This rule doesn't just apply to `asm!` but all of the macros.

This updates the inline assembly grammar description to use the grammar
style used in the rest of the reference, which also supports diagrams. I
also appreciate that this is more precisely defined using existing
grammar productions.

I realize that this is not as compressed as before. If that is a real
problem, then I think we can look at improving that throughout the
reference in general.

I did not change other parts of the chapter even though they mention
things like `in(<reg>) <expr>`. I could rewrite those, though I'm not
sure if that would help a lot.

There are restrictions not captured in the grammar which are explained
in the text. For example, global_asm and naked_asm do not allow
clobber_abi. I think I'm fine with the grammar not including everything
since there isn't the concept of an AST here.

This is not a direct translation because the old grammar was not
entirely precise or match with the existing grammar. This also renames
the rules to use PascalCase. The exhaustive list of differences:

Removed:
- Removed the `asm`, `naked_asm` and `global_asm` rules since I don't
  think they really added anything, and the macro names aren't exactly
  part of the grammar being described here. Instead it is just
  `AsmArgs`.
- Removed the explicit `_` since that is now a normal Expression.

New:
- FormatString added MacroInvocation because that is supported.
- Defined RegisterClass.
- Defined ExplicitRegister.

Changed:
- Renamed option and options to AsmOption and AsmOptions, and operand to
  AsmOperand (since our namespace is global).
- Clarified the meaning of "ident".
    - ArgName allows ident, keywords, and raw identifiers.
    - RegisterClass allows ident, keywords, but NOT raw identifiers.
- Split DirSpec into two, since there is a separate
  `Expression => Expression` syntax that is only valid for a subset
  of DirSpecs.
- Clarified that `sym` takes a PathExpression.
- Clarified that label's block doesn't allow inner attributes.
- Fixed sym/const/label which weren't properly quoted.
- Fixed some ambiguous precedence like alternation in reg_operand.
- Fixed `options` that it can be empty.