RFC: Add a matches!(expression, pattern) macro.

Rendered view: https://github.com/SimonSapin/rfcs/blob/matches-macro/active/0000-matches-macro.md

**Please note:** This is different from the [`if let`](https://github.com/rust-lang/rfcs/pull/160) proposal. I believe the two proposals are complementary rather than competing.

This is the `is` operator that was discussed at the `if let` proposal. 

The most reasonable version of it IMHO is that `is` is a keyword, and is an infix binary operator, where `EXPRESSION is PATTERN` is an expression of type `bool` and does basically the same thing as `matches!(EXPRESSION, PATTERN)`, and doesn't allow binding names.