Register snapshots.
auto merge of #18646 : eddyb/rust/snapshots, r=alexcrichton
 Implement low-hanging fruit of collection conventions

 * Renames/deprecates the simplest and most obvious methods
 * Adds FIXME(conventions)s for outstanding work
 * Marks "handled" methods as unstable

 NOTE: the semantics of reserve and reserve_exact have changed!
 Other methods have had their semantics changed as well, but in a
 way that should obviously not typecheck if used incorrectly.

 Lots of work and breakage to come, but this handles most of the core
 APIs and most eggregious breakage. Future changes should *mostly* focus on
 niche collections, APIs, or simply back-compat additions.

 [breaking-change]
fallout from conventions changes
test fixups