Port partition method from ~[T] to Vec<T>, for use early-late lifetime code.
Alpha-rename `.ident` fields of type `Name` to `.name`.
alpha-rename .ident to .name in Lifetime, including in rustdoc.
Introduce Scope<'a> shorthand for &'a ScopeChain<'a>.
Changed lists of lifetimes in ast and ty to use Vec instead of OptVec.

There is a broader revision (that does this across the board) pending
in #12675, but that is awaiting the arrival of more data (to decide
whether to keep OptVec alive by using a non-Vec internally).

For this code, the representation of lifetime lists needs to be the
same in both ScopeChain and in the ast and ty structures.  So it
seemed cleanest to just use `vec_ng::Vec`, now that it has a cheaper
empty representation than the current `vec` code.
Add proper support for early/late distinction for lifetime bindings.
Uses newly added Vec::partition method to simplify resolve_lifetime.
auto merge of #12807 : pnkfelix/rust/fsk-issue5121-fns-with-early-lifetime-params, r=pnkfelix

Fix issue #5121: Add proper support for early/late distinction for lifetime bindings.

There are some little refactoring cleanups as separate commits; the real meat that has the actual fix is in the final commit.

The original author of the work was @nikomatsakis; I have reviewed it, revised it slightly, refactored it into these separate commits, and done some rebasing work.