auto merge of #7996 : erickt/rust/cleanup-strs, r=erickt

This is a cleanup pull request that does:

* removes `os::as_c_charp`
* moves `str::as_buf` and `str::as_c_str` into `StrSlice`
* converts some functions from `StrSlice::as_buf` to `StrSlice::as_c_str`
* renames `StrSlice::as_buf` to `StrSlice::as_imm_buf` (and adds `StrSlice::as_mut_buf` to match `vec.rs`.
* renames `UniqueStr::as_bytes_with_null_consume` to `UniqueStr::to_bytes`
* and other misc cleanups and minor optimizations

diff --cc src/libextra/future.rs
Simple merge

diff --cc src/libextra/getopts.rs
Simple merge

diff --cc src/librustc/back/passes.rs
Simple merge

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

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/controlflow.rs
Simple merge

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

diff --cc src/librustc/middle/trans/glue.rs
index 024ab7af0814def9ef0f708d7960026c88a904e8,d66b43167cec110d194b587c50625fd2e51c01e8..75f553f397177fe6f8c9ffeec86309c64e08acad
--- 1/src/librustc/middle/trans/glue.rs
--- 2/src/librustc/middle/trans/glue.rs
+++ b/src/librustc/middle/trans/glue.rs
@@@ -37,11 -37,11 +37,10 @@@ use util::ppaux::ty_to_short_str
  
  use middle::trans::type_::Type;
  
 -use std::io;
  use std::libc::c_uint;
- use std::str;
  use syntax::ast;
  
 -pub fn trans_free(cx: block, v: ValueRef) -> block {
 +pub fn trans_free(cx: @mut Block, v: ValueRef) -> @mut Block {
      let _icx = push_ctxt("trans_free");
      callee::trans_lang_call(cx,
          langcall(cx, None, "", FreeFnLangItem),

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

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