Implement Index and IndexMut for Vec
deprecate Vec::get
auto merge of #15652 : nick29581/rust/vec_index, r=acrichto
librustc: Implement the fully-expanded, UFCS form of explicit self.

This makes two changes to region inference: (1) it allows region
inference to relate early-bound regions; and (2) it allows regions to be
related before variance runs. The former is needed because there is no
relation between the two regions before region substitution happens,
while the latter is needed because type collection has to run before
variance. We assume that, before variance is inferred, that lifetimes
are invariant. This is a conservative overapproximation.

This relates to #13885. This does not remove `~self` from the language
yet, however.

[breaking-change]
libsyntax: Remove `Send` from `PtrTy` in `deriving`.

It'll be complex to port to the new explicit-self regime and it seems to
be unused.
librustc: Allow the new UFCS explicit self in trait definitions, and
remove `~self` from the test suite.