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

src/libstd/unstable/intrinsics.rs

@@ -292,12 +292,6 @@ extern "rust-intrinsic" {
     /// elements.
     pub fn size_of<T>() -> uint;
 
-    /// Move a value to a memory location containing a value.
-    ///
-    /// Drop glue is run on the destination, which must contain a
-    /// valid Rust value.
-    pub fn move_val<T>(dst: &mut T, src: T);
-
     /// Move a value to an uninitialized memory location.
     ///
     /// Drop glue is not run on the destination.