Rename some methods in the os module

This pull request seems a bit bikeshedding, but I think it is necessary.

- `os::make_absolute()` -> `os::abspath()`: The name "make_absolute" is too generous to hold a specific meaning. At least it should contain "path". It seems that "abspath" is a common and concise name. (Also used in Python.)

- `os::change_dir()` -> `os::chdir()`: The method names in the os module generally follow the POSIX-like convention, such as setenv, getenv, getcwd, and errno. As os::change_dir() internally uses libc::chdir() on Unix, it would be a good idea to follow the same convention.