The "unescaped" rule was created by mechanically translating original
RFC 7159 rule:
unescaped = %x20-21 / %x23-5B / %x5D-10FFFF
into:
unescaped = [\x20-\x21\x23-\x5B\x5D-\u10FFFF]
However, this mechanical translation was incorrect as PEG.js grammars
don't have 6-digit Unicode escape sequences. Sequence "\u10FFFF" was
interpreted as "\u10FF" followed by two "F" characters.
This commit rewrites the "unescaped" rule into a form which, while not
being a mechanical translation of the original rule, matches the same
characters in the whole Unicode range. It also macthes textual
description of string representation in RFC 7159:
All Unicode characters may be placed within the quotation marks,
except for the characters that must be escaped: quotation mark,
reverse solidus, and the control characters (U+0000 through U+001F).
Fixes#417.
f07ab7f32e