Most notably this changes 'syntax::ext::base::get_single_str_from_tts'
to accept a trailing comma, and revises the documentation so that this
aspect is not surprising. I made this change under the understanding
that this crate is private rustc implementation detail (I hope this is
correct!). After reviewing all call sites, I believe the revised
semantics are closer to the intended spirit of the function.

Adds support for immovable generators. Move checking of invalid borrows across suspension points to borrowck. Fixes #44197, #45259 and #45093.

Avoid overlapping spans by only pointing at the arguments that are not
being used in the argument string. Enable libsyntax to have diagnostics
with multiple primary spans by accepting `Into<MultiSpan>` instead of
`Span`.

made `parser::Parser::expect_lifetime` public, so it can be called from `macro_parser::parse_nt`

Started rebasing @sgrif's PR #33135 off of current master. (Well, actually merging it into a new branch based off current master.)

The following files still need to be fixed or at least reviewed:

- `src/libsyntax/ext/tt/macro_parser.rs`: calls `Parser::parse_lifetime`, which doesn't exist anymore
- `src/libsyntax/parse/parser.rs`: @sgrif added an error message to `Parser::parse_lifetime`. Code has since been refactored, so I just took it out for now.
- `src/libsyntax/ext/tt/transcribe.rs`: This code has been refactored bigtime. Not sure whether @sgrif's changes here are still necessary. Took it out for this commit.