Simplify code generation and merge integral and expression generators

[IgorBaratta]

• Add shape to symbols
• Store all symbols in CodeRepresentation
• Use free functions for definitions and access
• Remove code for custom integrals (most of the code is untested and hasn't been updated for long time)

[coveralls]

coverage: 80.419% (-0.2%) from 80.657% when pulling 88ade31b35415a55e704021ba68cc7398f169a14 on igor/integral-expression into a67768cc2220e3690df85a63c9516aee2c9a7d24 on main.

[jhale]

This has moved quite far from main.

[IgorBaratta]

I'm closing this now. I've initiated work on a new branch, aiming for fewer modifications thanks to @jorgensd's PR.