Fuzzing setup and fixes (#755)

* encode: fix localdate formatting
* encode: fix empty key marshaling
* encode: fix invalid quotation of time.Time
* encode: ensure control chars are escaped
* decode: always use UTC for zero tz
* encode: check for invalid characters in keys
* encode: always construct map for empty array tables
* fuzz: add go 1.18 fuzz test
* encode: handle NaNs
* encode: allow new lines in quoted keys
* encode: never emit table inside array
* encode: don't capitalize inf

testdata/fuzz/FuzzUnmarshal/c87e8eb4e8fa40fd3c89d8ed16a049ce81592ded710965ee29a4e506eb5427b3

@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("0=0000-01-01 00:00:00+00:00")