Moves platform specific functionality into its own module.
Thanks to @Meralis40 for starting this!
This also gets just building on windows \^_^/
Although a lot of tests still fail (✖╭╮✖)
The `PlatformInterface` trait contains functions which abstract
over platform specific functionality, with implementations for
different platforms behind #[cfg(*)] attributes.
- `make_shebang_command()` constructs a command which will execute
the given script as if by a shebang. On linux this executes the
file, on windows it runs the interpreter directly.
- `set_execute_permission()` sets the execute permission on a
file. This is a nop on windows, since all files are executable.
- `signal_from_exit_status()` extracts the signal a process was
halted by from its exit status, if it was halted by a signal.
