Optional Rust speedups for Textual.

This module implements some of Textual's classes in Rust, which should make Textual apps faster. It is currently experimental, and shouldn't be used in production.

To use textual-speedups, install it in to the same environment as your Textual app:

pip install textual-speedups

Then run your Textual app with the environment variable TEXTUAL_SPEEDUPS=1. For example:

TEXTUAL_SPEEDUPS=1 python -m textual

Note, that you will need the current main version to enable the speedups.

The environment variable is there to discourage use in production. In the future when I am more confident in stability then the environment variable will be used to opt-out of the Rust speedups.

Currently, the classes in geometry.py have a Rust implementation. This includes Offset, Size, Region, and Spacing. These classes are used a lot internally when updating layout in particular.

That is yet to be determined. A little profiling suggests that pretty much all methods are several orders of magnitude faster than the pure-Python versions. This should add up to a respectable improvement, but until I've written a benchmark tool I won't be able to quantify that.

Note that unless your Textual app is particularly complex, you might not even notice a difference!

All tests pass with speedups enabled, and every app I have tested it against. However, it is possibly (even likely) there are edge cases remaining which may result in crashes or subtle differences.