Implement a proper mutex type for future usage

src/header/stdio/default.rs

src/header/stdio/helpers.rs

src/header/stdio/mod.rs

src/lib.rs

src/mutex.rs

src/platform/linux/mod.rs

src/platform/pal/mod.rs

src/platform/redox/mod.rs

tests/Makefile

tests/expected/stdio/mutex.stderr

tests/expected/stdio/mutex.stdout

tests/stdio/mutex.c