auto merge of #19113 : nikomatsakis/rust/unboxed-boxed-closure-unification, r=acrichto

Use the expected type to infer the argument/return types of unboxed closures. Also, in `||` expressions, use the expected type to decide if the result should be a boxed or unboxed closure (and if an unboxed closure, what kind).

This supercedes PR #19089, which was already reviewed by @pcwalton.

diff --cc src/librustc/middle/borrowck/mod.rs
Simple merge

diff --cc src/librustc/middle/resolve.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/librustc/middle/typeck/astconv.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/mod.rs
Simple merge

diff --cc src/librustc_back/svh.rs
Simple merge

diff --cc src/librustc_trans/trans/base.rs
Simple merge

diff --cc src/librustc_trans/trans/expr.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/feature_gate.rs
Simple merge

diff --cc src/libsyntax/fold.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge

diff --cc src/libsyntax/visit.rs
Simple merge