Add macro_rules fragment class for "expressions that can be followed by `{`"

(spawned off of https://github.com/rust-lang/rfcs/pull/1384#issuecomment-160165525 )

I've been leaving links to the above comment in all sorts of places, so I finally decided it was time to open an RFC issue for this idea.

Namely, to add some fragment class, which I'll call pred for now, that represents "the subset of expressions that do not have { in their follow set, and thus the presence of a { as the next token tells the parser that it is definitely done parsing the expression."

In particular, this is the subset of expressions that we use for the test in an if test { then } else { other }

Such an fragment class would be useful for writing macro_rules! that naturally want a { to come after them in the input pattern.

We used to superficially "allow" such macros, but they didn't actually work in general; see https://github.com/rust-lang/rust/issues/26739

Start by reading the linked RFC discussion and the referenced rust-lang/rust issue 26739 to understand the prior behavior and constraints. Done means reaching agreement on the proposed macro_rules fragment class and documenting the design sufficiently for an RFC or implementation to proceed.