Add filesystem operations from `base` to `pathway-system`

#14 covers implementing the directory API, but there are a number of operations in base that should also be replicated.

For example, System.IO has withFile, openFile, openTempFile, etc., which should also have Path-based versions.

However, System.IO is a significant module, and some approach to avoid conflicts needs to be considered. Some options:

• expect the Pathway modules to be imported qualified (but that leaves the “dangerous” base versions available unqualified);
• re-export the entire module, creating an entire replacement for System.IO; or
• re-modularize the rest of the module, putting the FilePath operations in Filesystem.Path, and putting the rest into one or more other modules, with the expectation that they will be used instead of System.IO.

The actual implementation of the operations largely involves un/wrapping the FilePaths, and should follow pathway-system:Filesystem.Path’s existing approach.