* removed a putenv() implementation which isn't needed anymore
* mch_getenv() and mch_setenv() are now functions in src/os/env.c
* removes direct calls to getenv() and setenv() outside of src/os/env.c
* refactored the logic of get_env_name into mch_getenvname_at_index
* added unittests for the functions in os/env.c
* Rename mch_full_name to mch_get_absolute_path.
* Rename mch_is_full_name to mch_is_absolute_path.
* Add a lot of missing parentheses.
* Remove yoda-conditions for consistency.
* Remove spaces in function declaration.
* Rename mch_FullName to mch_full_name to match the style guide.
* Add mch_full_dir_name, which saves the absolute path of a given
directory relative to cwd into a given buffer.
* Add function append_path, which glues together two given paths with a
slash.
* Adapt moonscript coding style to the tests.
