Remove `ObsoleteInPlace`
The in place syntax has been deprecated for over a year. As it is, this is accumulated cruft: the error messages are unlikely to be helpful any more and it conflicts with some useful syntax (e.g. const generics in some instances).
It may be that removing `Token::LArrow` is backwards-incompatible. We should do a crater run to check.
cc @eddyb
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the
rustc guide: