convert doc-attributes to doc-comments using ./src/etc/sugarise-doc-comments.py (and manually tweaking) - for issue #2498

Revert "Moved arc to libstd and added an arc that allows shared mutable state through mutual exclusion."

This reverts commit 015527b0.

Needs more tests to ensure safety, and probably some more work on usability too.

.. turns out that, without the export, the modules aren't accessible
outside of the crate, itself. I thought that, by importing some module
into another (nesting it) and exporting from that nested module (which
is, itself, exported from std.rc) that my mod would be in the build
artifact. This doesn't appear to be the case. learning is fun!

This should go in libstd, but currently resolve bugs make this not
work.

.. leveraging std::uv, we have:
timer::delayed_send - send a value over a provided channel after the
timeout has passed
timer::sleep - block the current task for the specified period

both of these fns (and everything that goes in timer.rs) leverage the
uv_timer_* API

- moved global loop tests, as well.. will add tests in uv_hl that encompass
rolling your own high_level_loop via uv::hl::run_high_level_loop()
- also whitespace cleanups and misc warning cleanup..
- doesn't work on 32bit linux

Closes #1892

 - Move io, run and rand to core.
 - Remove incorrect ctypes module (use libc).
 - Remove os-specific modules for os and fs.
 - Split fs between core::path and core::os.

- removing the remains of uvtmp.rs and rust_uvtmp.rs
- removing the displaced, low-level libuv bindings
in uv.rs and rust_uv.cpp