rt::io: Add Bytes iterator for Reader

An iterator that simply calls `.read_bytes()` each iteration.

I think choosing to own the Reader value and implementing Decorator to
allow extracting it is the most generically useful. The Reader type
variable can of course be some kind of reference type that implements
Reader.
rt::io: Rename Bytes to ByteIterator and add note about iteration
Rename MutexArc access methods to unsafe_access
Add a safe implementation of MutexArc::access* methods

Current access methods are nestable and unsafe. This patch renames
current methods implementation - prepends unsafe_ - and implements 2 new
methods that are both safe and un-nestable.

Fixes #7473
Fixed docs and styles
Use MuextArc and RWArc in docstrings
Added explicit pub to several conditions.  Enables completion of #6009.
Make non-pub condition! expand to non-pub mod.  Fix #6009.
Convert rust_log.cpp to Rust, closes #8703
auto merge of #8966 : FlaPer87/rust/issue/7473, r=bblum

Current access methods are nestable and unsafe. This patch renames
current methods implementation - prepends unsafe_ - and implements 2 new
methods that are both safe and un-nestable.

Fixes #7473
libstd/os: set tmp dir to /data/tmp on Android

Android has no /tmp partition, return /data/tmp instead. Cf. #8511.
Another followup on #6009.

Odd that my earlier make checks did not catch this.
debuginfo: Support for variables captured in closures and closure type descriptions.
debuginfo: Support for by-value arguments (still excluding some cases of self arguments)
debuginfo: Added test cases for structs, tuples, enums, etc passed by value.

Also updated documentation comments in debuginfo and renamed DebugContext to CrateDebugContext.
debuginfo: Always copy args to allocas if debuginfo is enabled
debuginfo: Fixed some merge fallout
debuginfo: Much improved handling of captured variables and by-value arguments.
auto merge of #8901 : adridu59/rust/issue-8511, r=huonw

Android has no /tmp partition, cf. #8511.
debuginfo: Fixed some merge fallout