Add `src` property to Module for control over I/O

[ericf]

This splits apart reading in the module src code from parsing it into an AST. By adding a src property to Module instances, there's now a hook to control file I/O — for instance to cache the src of unchanged modules between re-builds.

[caridy]

:100:

[ericf]

@eventualbuddha thanks for merging this. If you don't mind, it would be great to get this out in a v0.6.1 release.

[eventualbuddha]

@ericf done.

[ericf]

Thanks!