Cleanup, document, centralize wordLang syntactic conventions

[tanyongkiam]

At present, wordLang passes establish/preserve a number of syntactic properties across different phases.

These properties have gotten a bit too much to handle for adding new passes (one often needs to prove several syntactic properties completely unrelated to the semantic correctness proof).

This issue is about taking a first step towards fixing this.

I suggest to create two files:

semantics/word_convTheory which will store all the definitions of syntactic conventions that need to be preserved ACROSS passes in wordLang. Most (or all?) of these are currently in semantics/wordPropsTheory so this is a matter of moving them.

proofs/word_convProofTheory which will store the individual proof of the syntax properties for every pass.

The idea is that, for a new compiler pass, it should be hopefully relatively easy to test that we have met all the necessary syntactic conventions by building word_convProofTheory.