EBNF formatting

[jesse99]

I find the EBNF rules in the manual rather hard to read. Here are a few problems:

• Everything is space separated so there are no visual cues about what belongs with what. This is especially problematic with the * and ? meta-characters.
• Single quotes are used for literal characters and double quotes for literal strings. There doesn't seem much point in this and using just single quotes would be less visually distracting.
• It seems odd to use square brackets for grouping.
• Some typographic conventions would be helpful. Maybe meta characters in bold.

For example compare:
vec_expr : '[' "mut" ? [ expr [ ',' expr ] + ] ? ']'

with:
vec_expr : '[' 'mut'? ( expr ( ',' expr ) + ) ? ']'

(Too many spaces in the above because markdown gets confused with series of asterisks).

[graydon]

We're going to be completely replacing the grammar rules with rules extracted from a machine-checked grammar. Likely they'll be in whatever format the machine-checking tool uses. Closing this as WONTFIX; we may revisit typographic issues when the grammar gets refreshed.