Implementing custom Span type.

[emilyaherbert]

Closes #152.

This PR implements a custom span type that allows the Span to keep track of the filename. This is useful because it could allow for users to see more descriptive error messages (#214 #73 ). As a biproduct of this change, the error types in the compiler had to change as well, with any &'sc str components changing to String components, among other small changes.

Summary

1. Implement custom span type Span
2. Have all parsing methods use Span

[sezna]

Looks pretty solid so far, exactly what I was envisioning 😄

[sezna]

Could you clarify why we need to change all of our &'sc strs to Strings? This is a pretty significant change and introduces a lot of malloc overhead to our error/warning system. I'm not immediately seeing why we need to do this.

[sezna]

LGTM, although, for future reference, we should store a &'manifest PathBuf instead of a PathBuf to avoid all the allocations.