core: a few core::os fns ported to rust + ptr::array_each

[olsonjeffery]

Happy Presidents' Day!

Four things:

• added ptr::array_each() to core, used to iterate over the elements in a **T. ptr::array_each assumes the provided array is null-terminated and determins length via ptr::buf_len. There is a ptr:array_each_with_len that takes a length parameter
• ported os::get_cwd() to rust, calling into C library fns in core::libc
• ported os::env() to rust. The win32 version uses API available in libc::funcs::extra::kernel32, while the _nix versions still use the rust_env_pairs in rt, which now just return the _*environ (in platform applicable fashion).
• porting os::list_dir() to rust (unix is complete, windows complete)

The latter three are towards finishing #4812 .

core tests are passing in linux and OSX. I have an issue with my windows vm where rustc chews up and keeps hitting the 32bit process memory limit before segfaulting on one of the stage1 library builds. But I know that libcore builds on win32.

[olsonjeffery]

bleh. i hazarded to rebase against incoming based on a mistaken impression that history would be preserved.. but obviously not.. it's been blown away. apologies for the inconvenience, won't rebase again.

working on getting os:list_dir() finished.. that'll remove the last of the make_str / kernel->malloc's / etc from rust_builtin.cpp.

[olsonjeffery]

adding ptr::array_each() is done as a part of c5c60c8

[brson]

I'll fix that and try again. I want those windows test fixes.