Support transforms with multiple arguments

[SuprDewd]

This adds support for transforms with multiple arguments, e.g. f = 1+x*DIRICHLET(f,{1,...}).

It's still a work in progress (although things seem to work). In particular:

• [x] Some tests seem to be failing.
• [ ] Tests should be added for the changes.
• [x] Documentation (e.g. grammar) needs to be updated.
• [x] Make sure "Hops as a library" isn't broken.
• [x] Possibly clean up the implementation.

[akc]

That's a very nice addition. I'm sure it will be useful for other functions as well (besides DIRICHLET).

I just made a few cosmetic changed (most of which were suggested by hlint).