Lowering doc-comments

This phase of processing converts an input sequence of fine-grained tokens to a new sequence of fine-grained tokens.

The new sequence is the same as the input sequence, except that each LineComment or BlockComment token whose style is inner doc or outer doc is replaced with the following sequence:

• Punctuation with mark #
• Whitespace
• Punctuation with mark ! (omitted if the comment token's style is outer doc)
• Punctuation with mark [
• Identifier with represented identifier doc
• Punctuation with mark =
• Whitespace
• RawStringLiteral with the comment token's body as the represented string and empty suffix
• Punctuation with mark ]

Note: the whitespace tokens aren't observable by anything currently described in this writeup, but they explain the spacing in the tokens that proc-macros see.