Auto merge of #40043 - petrochenkov:objpars, r=nikomatsakis

Refactor parsing of trait object types

Bugs are fixed and code is cleaned up.

User visible changes:
- `ty` matcher in macros accepts trait object types like `Write + Send` (https://github.com/rust-lang/rust/issues/39080)
- Buggy priority of `+` in trait object types starting with `for` is fixed (https://github.com/rust-lang/rust/issues/39317). `&for<'a> Trait<'a> + Send` is now parsed as `(&for<'a> Trait<'a>) + Send` and requires parens `&(for<'a> Trait<'a> + Send)`. For comparison, `&Send + for<'a> Trait<'a>` was parsed like this since [Nov 27, 2014](https://github.com/rust-lang/rust/pull/19298).
- Trailing `+`s are supported in trait objects, like in other bounds.
- Better error reporting for trait objects starting with `?Sized`.

Fixes https://github.com/rust-lang/rust/issues/39080
Fixes https://github.com/rust-lang/rust/issues/39317 [breaking-change]
Closes https://github.com/rust-lang/rust/issues/39298
cc https://github.com/rust-lang/rust/issues/39085 (fixed, then reverted https://github.com/rust-lang/rust/pull/40043#issuecomment-286570653)
cc https://github.com/rust-lang/rust/issues/39318 (fixed, then reverted https://github.com/rust-lang/rust/pull/40043#issuecomment-284493890)

r? @nikomatsakis