[qodelicious]

Fixes #614

Updates docs/grammar.md so it matches what the parser actually accepts. I checked each case against Deserializer::from_str + IgnoredAny + end() on master.

• Strings and byte strings. A \ always starts an escape, so "a\c" is rejected. The plain character class now excludes the backslash too.
• Unicode escapes need braces, so \u{41} works and \u0041 doesn't. Also added a note that it takes 1 to 6 hex digits and has to be a valid scalar value.
• Chars take the same escapes as strings ('\n', '\x41', '\u{41}' all parse), just without line continuations, and a \x escape in a char has to be ASCII.
• Ranges don't allow whitespace around .. or ..= (1 .. 2 and .. 5 are rejected), so I dropped the ws from those rules.
• Whitespace and comments are allowed before #![enable(...)], so RON now starts with ws.

I left out the bare ''' char and the 1e_+0 exponent cases from the issue, since they look more like parser quirks than something the grammar should promise.

• I've included my change in CHANGELOG.md

[juntyr]

Where are no_double_quotation_marks_or_backslash or no_apostrophe_or_backslash defined? If they aren't yet, they should be given an explicit rule similar to nested_block_comment