switch to meson build

This is a work in progress effort to modernize our build machinery and use meson, recently adopted by the scipy stack.

In doing this I also follow the approach adopted by the scipy stack of using Tempita, Cython's built-in templating engine.

Rough plan:

• Change the build infrastructure to use meson-python and pyproject.toml
• Implement environment-dependent search for mumps (right now I target conda-forge) Library names can now be passed via a meson build option.
• Figure out the development workflow (running tests etc)
• Update CI
• Explore conda packaging

Also closes #19 (closed) and #11 (closed) (I cleaned up a bunch of code for a good measure).