Simplify classes that could be implemented with `[BottomUp]TermTransformer`

[mezpusz]

The codebase seemingly has some instances of term transforming where we could use TermTransformer or BottomUpTermTransformer.

For example:

• SymbolDefinitionInlining
• SymbolOccurrenceReplacement

Hunt these down and simplify them to remove hard-to-maintain term transformations.

[MichaelRawson]

#376 related but not entirely the same. Let's try to do BUE if we can, I find it more intuitive.

[mezpusz]

Ah, right! We can use whichever, as long as the codebase shrinks. 🙂