Remove date regexp (#447)

* Remove date regexp

Hand-roll the date matching logic to avoid trying to match a regexp on
every integer.

```
benchmark                    old ns/op     new ns/op     delta
BenchmarkUnmarshalToml-8     293449        272134        -7.26%

benchmark                    old allocs     new allocs     delta
BenchmarkUnmarshalToml-8     2746           2650           -3.50%

benchmark                    old bytes     new bytes     delta
BenchmarkUnmarshalToml-8     133604        127548        -4.53%
```

* Remove fuzzit

The company has been acquired by GitLab and shutting down.

benchmark/benchmark_test.go

@@ -137,6 +137,7 @@ func BenchmarkUnmarshalToml(b *testing.B) {
 	if err != nil {
 		b.Fatal(err)
 	}
+	b.ReportAllocs()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		target := benchmarkDoc{}