Upstream language-rust modifications

[cronburg]

Look into getting the antiquotation support in the Rust parser integrated into the upstream language-rust repository.

This can look almost exactly like what this project does now to modify the parser, which is to create a separately named quasiquoter which tells the generated yacc parser (Parser.y?) to enable or disable the antiquotation feature.

The complicating factor is that antiquotation also modifies the lexer to support dollar-sign variable names without the more verbose quasiquotation syntax. So I have to figure out a way to swap out lexers at Haskell runtime. I vaguely remember lex / happy supporting this in Haskell, where you manually write the Haskell function which iteratively scans for new tokens in the input.

[cronburg]

language-rust could also benefit from support for parsing and printing of crate documentation, and comments in general. The rustdoc command is of interest here.

For the purposes of Floorplan, what we need is:

• A way to splice in variable names (with dollar signs) directly in rustdoc comments.
• Preservation of whitespace and style inside of \\\ rustdoc comments.
• A way to splice in arbitrary strings into the docs (to, e.g., refer directly to the piece of the Floorplan spec responsible for generating a piece of code).