move to more performant internal representation

The current internal representation leaves a lot to be desired.

Low hanging fruit would be to directly represent a(a(a(a(b)))) not as 5 different functions, but maybe just one function with a `composed: 4` key. That would probably make performance actually acceptable for large functions.

The final version of this is that we can probably get hella hella speed improvements if we 'compile' the expression before beta reduction... but that sounds hard.

Requires having tests