In-language documentation

[jkarni]

• There should be a doc function which, given an identifier, returns the documentation for that identifier (if it exists).
• There should be a way of documenting new functions that integrates with that doc function.
• Existing primops should be documented.

I initially imagined there was good reason to have doc work only in the impure environment (since we want to maintain referential transparency). But I now realize doc can work on quoted identifiers and maintain referential transparency. (Note that quote is not a function, and in typed settings at least for subtle and profound reasons that I only half-remember, so there's no function from an unquoted value to it's documentation.)

[jameshaydon]

Fixed in #177.