Remove @ast::Region and replace with @ast::Lifetime.
Modify pretty-printer to emit lifetimes and fix a few minor
parser bugs that this uncovered.
auto merge of #5296 : nikomatsakis/rust/region-syntax-expl-lifetimes-2, r=nikomatsakis

Modify pretty-printer to emit lifetimes and fix a few minor
parser bugs that this uncovered.

r? whomever.