auto merge of #8532 : kballard/rust/cstr-cleanup, r=erickt

Implement interior null checking in `.to_c_str()`, among other changes.

diff --cc src/librustc/middle/trans/base.rs
Simple merge

diff --cc src/librustc/middle/trans/common.rs
Simple merge

diff --cc src/librustc/middle/trans/consts.rs
Simple merge

diff --cc src/librustc/middle/trans/context.rs
Simple merge

diff --cc src/librustc/middle/trans/controlflow.rs
Simple merge

diff --cc src/librustc/middle/trans/expr.rs
Simple merge

diff --cc src/librustpkg/util.rs
Simple merge

diff --cc src/libstd/ptr.rs
Simple merge

diff --cc src/libstd/run.rs
Simple merge