URL Imports

[peblair]

Currently, all modules need to be on the local Grain PATH in order for compilation to take place. I would like imports to be able to be specified by a URL as well as the current import scheme.

I'm thinking this can play into a larger and more extensible Racket-like import system, in which one can write a custom import specifier; however, this would require support for either compiler plugins or macros.