Document the synom parser macros

[mystor]

This documents the rest of the parser macros, fixing #87.

I did it all in one go, and I didn't read over it, so there may be some mistakes. Please point them out when you look over it.

[dtolnay]

Awesome! I made it partway through and I will continue tomorrow.

One thing I realized i need to add myself is an overview of the whitespace-handling strategy. I think the way we do it is far better (for our use case) than nom's ws macro. In short: we have a small number of building-block parsers (punct, keyword, lit, doc comment) that all ignore leading whitespace. As long as every other parser is combinated out of only these building blocks, nothing else needs to worry about whitespace anywhere else. And it is great for compile time - the cost in compile time that we pay for correct whitespace handling is basically four function calls, one in each building-block parser, rather than pervasive match expressions checking for whitespace between every possible pair of tokens like ws would do.