auto merge of #11585 : nikomatsakis/rust/issue-3511-rvalue-lifetimes, r=pcwalton

auto merge of #11585 : nikomatsakis/rust/issue-3511-rvalue-lifetimes, r=pcwalton

Major changes:

- Define temporary scopes in a syntax-based way that basically defaults
  to the innermost statement or conditional block, except for in
  a `let` initializer, where we default to the innermost block. Rules
  are documented in the code, but not in the manual (yet).
  See new test run-pass/cleanup-value-scopes.rs for examples.
- Refactors Datum to better define cleanup roles.
- Refactor cleanup scopes to not be tied to basic blocks, permitting
  us to have a very large number of scopes (one per AST node).
- Introduce nascent documentation in trans/doc.rs covering datums and
  cleanup in a more comprehensive way.

r? @pcwalton