Remove some allocs for slices in interfaces

```
name                               old time/op    new time/op    delta
UnmarshalDataset/config-2            24.9ms ± 1%    24.9ms ± 0%     ~     (p=0.413 n=5+4)
UnmarshalDataset/canada-2            66.1ms ± 0%    61.7ms ± 1%   -6.63%  (p=0.008 n=5+5)
UnmarshalDataset/citm_catalog-2      25.3ms ± 5%    24.7ms ± 1%   -2.09%  (p=0.032 n=5+5)
UnmarshalDataset/twitter-2           10.9ms ± 2%    10.9ms ± 2%     ~     (p=1.000 n=5+5)
UnmarshalDataset/code-2               108ms ± 0%     108ms ± 0%     ~     (p=0.095 n=5+5)
UnmarshalDataset/example-2            177µs ± 2%     176µs ± 0%     ~     (p=0.841 n=5+5)
Unmarshal/SimpleDocument/struct-2     579ns ± 0%     586ns ± 1%   +1.30%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/map-2        875ns ± 1%     876ns ± 0%     ~     (p=0.548 n=5+5)
Unmarshal/ReferenceFile/struct-2     49.7µs ± 1%    49.5µs ± 0%     ~     (p=0.095 n=5+5)
Unmarshal/ReferenceFile/map-2        80.4µs ± 0%    79.6µs ± 0%   -0.99%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-2          13.9µs ± 0%    13.7µs ± 0%   -1.70%  (p=0.008 n=5+5)

name                               old speed      new speed      delta
UnmarshalDataset/config-2          42.1MB/s ± 1%  42.2MB/s ± 0%     ~     (p=0.381 n=5+4)
UnmarshalDataset/canada-2          33.3MB/s ± 0%  35.7MB/s ± 1%   +7.11%  (p=0.008 n=5+5)
UnmarshalDataset/citm_catalog-2    22.1MB/s ± 5%  22.6MB/s ± 1%   +2.08%  (p=0.032 n=5+5)
UnmarshalDataset/twitter-2         40.7MB/s ± 2%  40.6MB/s ± 2%     ~     (p=1.000 n=5+5)
UnmarshalDataset/code-2            24.8MB/s ± 0%  24.9MB/s ± 0%     ~     (p=0.103 n=5+5)
UnmarshalDataset/example-2         45.8MB/s ± 2%  46.0MB/s ± 0%     ~     (p=0.841 n=5+5)
Unmarshal/SimpleDocument/struct-2  19.0MB/s ± 0%  18.8MB/s ± 1%   -1.26%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/map-2     12.6MB/s ± 1%  12.6MB/s ± 0%     ~     (p=0.508 n=5+5)
Unmarshal/ReferenceFile/struct-2    105MB/s ± 1%   106MB/s ± 0%     ~     (p=0.095 n=5+5)
Unmarshal/ReferenceFile/map-2      65.2MB/s ± 0%  65.8MB/s ± 0%   +1.00%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-2        39.3MB/s ± 0%  40.0MB/s ± 0%   +1.73%  (p=0.008 n=5+5)

name                               old alloc/op   new alloc/op   delta
UnmarshalDataset/config-2            5.85MB ± 0%    5.85MB ± 0%   -0.00%  (p=0.008 n=5+5)
UnmarshalDataset/canada-2            76.6MB ± 0%    75.2MB ± 0%   -1.76%  (p=0.016 n=4+5)
UnmarshalDataset/citm_catalog-2      35.3MB ± 0%    35.0MB ± 0%   -0.71%  (p=0.008 n=5+5)
UnmarshalDataset/twitter-2           13.5MB ± 0%    13.5MB ± 0%   -0.19%  (p=0.016 n=4+5)
UnmarshalDataset/code-2              22.3MB ± 0%    22.0MB ± 0%   -1.31%  (p=0.008 n=5+5)
UnmarshalDataset/example-2            204kB ± 0%     203kB ± 0%   -0.34%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-2      709B ± 0%      709B ± 0%     ~     (all equal)
Unmarshal/SimpleDocument/map-2       1.08kB ± 0%    1.08kB ± 0%     ~     (all equal)
Unmarshal/ReferenceFile/struct-2     19.8kB ± 0%    19.7kB ± 0%   -0.24%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-2        37.3kB ± 0%    37.0kB ± 0%   -0.64%  (p=0.029 n=4+4)
Unmarshal/HugoFrontMatter-2          7.26kB ± 0%    7.22kB ± 0%   -0.66%  (p=0.008 n=5+5)

name                               old allocs/op  new allocs/op  delta
UnmarshalDataset/config-2              230k ± 0%      230k ± 0%   -0.00%  (p=0.000 n=5+4)
UnmarshalDataset/canada-2              447k ± 0%      391k ± 0%  -12.53%  (p=0.008 n=5+5)
UnmarshalDataset/citm_catalog-2        169k ± 0%      158k ± 0%   -6.20%  (p=0.029 n=4+4)
UnmarshalDataset/twitter-2            55.8k ± 0%     54.7k ± 0%   -1.88%  (p=0.029 n=4+4)
UnmarshalDataset/code-2               1.06M ± 0%     1.05M ± 0%   -1.14%  (p=0.008 n=5+5)
UnmarshalDataset/example-2            1.31k ± 0%     1.28k ± 0%   -2.21%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-2      8.00 ± 0%      8.00 ± 0%     ~     (all equal)
Unmarshal/SimpleDocument/map-2         13.0 ± 0%      13.0 ± 0%     ~     (all equal)
Unmarshal/ReferenceFile/struct-2        125 ± 0%       123 ± 0%   -1.60%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-2           600 ± 0%       590 ± 0%   -1.67%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-2             132 ± 0%       130 ± 0%   -1.52%  (p=0.008 n=5+5)
```

unmarshaler.go

@@ -625,7 +625,7 @@ type unmarshalArrayFn func(d *decoder, array *ast.Node, v reflect.Value) error
 
 var globalUnmarshalArrayFnCache atomic.Value // map[danger.TypeID]unmarshalArrayFn
 
-func unmarshalArrayFnFor(vt reflect.Type) unmarshalArrayFn {
+func unmarshalArrayFnForSlice(vt reflect.Type) unmarshalArrayFn {
 	tid := danger.MakeTypeID(vt)
 
 	cache, _ := globalUnmarshalArrayFnCache.Load().(map[danger.TypeID]unmarshalArrayFn)
@@ -693,7 +693,7 @@ func (d *decoder) unmarshalArray(array *ast.Node, v reflect.Value) error {
 	switch v.Kind() {
 	case reflect.Slice:
 		vt = v.Type()
-		fn := unmarshalArrayFnFor(vt)
+		fn := unmarshalArrayFnForSlice(vt)
 		return fn(d, array, v)
 	case reflect.Array:
 		vt = v.Type()
@@ -701,8 +701,8 @@ func (d *decoder) unmarshalArray(array *ast.Node, v reflect.Value) error {
 	case reflect.Interface:
 		elem := v.Elem()
 		if !elem.IsValid() {
-			elem = reflect.New(sliceInterfaceType).Elem()
+			s := make([]interface{}, 0, 16)
-			elem.Set(reflect.MakeSlice(sliceInterfaceType, 0, 16))
+			elem = reflect.ValueOf(&s).Elem()
 		} else if elem.Kind() == reflect.Slice {
 			if elem.Type() != sliceInterfaceType {
 				elem = reflect.New(sliceInterfaceType).Elem()