This adds a new `env` module with `override_temp_dir` and `temp_dir`
functions.

- `temp_dir` will defer to `std::env::temp_dir` by default unless the
temporary directory has been overridden.
- `override_temp_dir` allows the user to override the default temporary
directory ONCE. Once this value has been set, it cannot be changed Care
should be taken to ensure that the chosen directory is actually writable.

This API is designed for use by the application author when the
application may run in an environment without a reliable global
temporary directory (e.g., Android).

fixes #285