Auto defining extensions with attached lowerings

[ss2165]

The compiler already features the @guppy.custom pattern for marking a function as lowering to an existing hugr extension.

It would be useful to be able to declare types and functions that are mapped to a generated hugr extension. Compilation would produce a HUGR defined over the new extension and an extra module containing the definitions that can be used to link/lower those operations.

This is a convenient and quick way to define useful compile-time abstractions that can be removed further down the compilation stack.

[ss2165]

Pauli exponentials are a good example to start with