This adds the ability to specify Unicode code points in a character
range. This will be useful for defining some productions without using
English, and perhaps to be a little clearer.

This also extends the Unicode grammar to allow up to 6 characters for
larger code points.

This replaces some suffixes and prose with the new negative lookahead
syntax instead. This should all have the same meaning.

This clarifies that bare `//` is explicitly meant to be either followed
by LF or EOF. Otherwise it incorrectly matches other comment rules.

This fixes the BLOCK_COMMENT grammar so that it follows the rule that
the first alternation that matches wins. The previous grammar would fail
with the use of the cut operator to parse these two forms.

This fixes the doc comments so that they properly handle a carriage
return by using the cut operator. Rustc will fail parsing if a doc
comment contains a carriage return.

This requires including (LF|EOF) at the end of line so the cut operator
has something to complete the line.

This also removes the negative `/` from OUTER_LINE_DOC. This does not
work correctly with the check for CR, and is not needed because
LINE_COMMENT already matches `////`. Later I plan to include a rule for
comments that makes it clear the order that they are parsed.

A negative lookahead is necessary in OUTER_BLOCK_DOC to prevent it from
trying to parse what should be a BLOCK_COMMENT as an OUTER_BLOCK_DOC and
failing due to the cut operator.

This is intended to indicate the order that the rules are expected to be
processed (as defined in this grammar). Of course real parsers can take
a different approach if they have the same results.

This is roughly similar to the order that rustc takes, though
[`block_comment`](https://github.com/rust-lang/rust/blob/d7daac06d87e1252d10eaa44960164faac46beff/compiler/rustc_lexer/src/lib.rs#L782-L817)
roughly takes the approach of combining the `/*` prefix, and then
deciding if it is an inner doc comment, outer doc comment, or else a
regular block comment.

LINE_COMMENT must be first so that it is not confused with a doc
comment.

BLOCK_COMMENT must be last so that its cut operator does not interfere
with doc comments that start with `/*`. It could be moved up higher in
the list if it had negative lookahead to disambiguate OUTER_BLOCK_DOC,
but the expression for that is more complicated than the one in
OUTER_BLOCK_DOC.

rustc actually includes the spaces for doc comments.

The cut operator after (`e`|`E`) in `FLOAT_EXPONENT` reflects rustc's
actual parsing behavior: once the lexer sees an exponent indicator, it
commits and does not backtrack.  This makes the last `RESERVED_NUMBER`
alternative -- which existed to catch the empty-exponent case --
redundant, since the cut in `FLOAT_EXPONENT` now handles it directly.

Co-authored-by: Eric Huss <eric@huss.org>

The description says characters can be "surrounded in
backticks", but it'd be better to say "surrounded by".

The grammar now accepts 4-6 hex digits for Unicode code points (needed
for values above U+FFFF), so let's update the notation column to
reflect the variable width.  Let's also capitalize "Unicode", which is
a proper noun.

These tests cover:

- Parser: negative lookahead with nonterminals, terminals, charsets,
  grouped expressions, within sequences, repetitions, and
  alternations; error case for trailing `!`; Unicode code points with
  4, 5, and 6 hex digits; charset ranges with `Character::Char`,
  `Character::Unicode`, and mixed forms; charsets combining named
  entries, terminals, and Unicode ranges.

- Markdown renderer: negative lookahead rendering with `!`, Unicode
  rendering as `U+xxxx`, charset rendering with char and Unicode
  ranges, cut and neg expression rendering, and markdown escaping.

- Railroad renderer: negative lookahead renders as a "not followed by"
  labeled box, Unicode renders as terminal, charset ranges, cut
  renders as "no backtracking" labeled box, and neg expression renders
  as "with the exception of" labeled box.