Improve UTF-8 decoding and encoding functions (#410)

Ensure proper UTF-8 encoding (1 to 4 bytes).
Handle invalid encodings (return 0xFFFD and consume a single byte)
Individually encoded surrogate code points are accepted.

- add `utf8_scan()` to analyze a byte array for UTF-8 contents
  detects invalid encoding, computes number of codepoints and content kind:
  plain ASCII, 8-bit, 16-bit or larger codepoints.
- add `utf8_encode_len(c)` to compute the number of bytes to encode `c`
- rename `unicode_to_utf8` as `utf8_encode`
- rename `unicode_from_utf8` as `utf8_decode`
- add `utf8_decode_buf8(dest, size, src, len)` to decode a UTF-8 encoded
  byte array known to contain only ASCII and 8-bit codepoints.
- add `utf8_decode_buf16(dest, size, src, len)` to decode a UTF-8 encoded
  byte array into an array of 16-bit codepoints using UTF-16 surrogate pairs
  for non-BMP1 codepoints.
- add `utf8_encode_buf8(dest, size, src, len)` to encode an array of 8-bit
  codepoints as a UTF-8 encoded null terminated string
- add `utf16_encode_buf8(dest, size, src, len)` to decode an array of 16-bit
  codepoints (including surrogate pairs) as a UTF-8 encoded null terminated string
- detect invalid UTF-8 encoding in RegExp parser
- simplify `JS_AtomGetStrRT`, `JS_NewStringLen` using the above functions
- simplify UTF-8 decoding and error testing