Change how keywords are handled

[kevinbarabash]

Instead of parsing things as keywords without knowing whether the token needed is a keyword or not, let's instead have a parse_keyword(keyword: &str) method that calls as a parse_ident() method and then checks if the return identifier matches the expected keyword.

[kevinbarabash]

This turned out to be harder than it looked. I'll probably have to create a small toy parser to see if I can figure out the right pattern before implementing it for real.