Add resolve() method to SourceProvider

[dgp1130]

This adds a new resolve() method to SourceProvider which is responsible for resolving an @import specifier into a file path. FileProvider still uses the default behavior of assuming the specifier is a relative path and joining it with the path of the originating file. This provides a hook for custom SourceProviders to implement their own application-specific resolution logic.

This required a couple minor refactors in the fs! and error_test() utilities to make them more flexible and support testing a custom resolver.

I've done a little Rust, but I'm not terribly familiar with it, so please let me know if I'm doing anything unidiomatic. Some of the areas I struggled with and am not convinced I found the ideal solution:

1. error_test() accepts a callback which runs assertions. This was the only way I could reuse the function to assert different error kinds, since the only way to compare the BundleErrorKind enum is with matches!(), and a match expression can't really work as a function input. I tried to return the error instead, but that ran into ownership issues since it outlived the bundler, and I couldn't find a good solution there.
2. I wanted the bundler to automatically prepend any resolver errors with "Failed to resolve {}:\n...", so that the context of the failure is always present, even if the resolver itself gave a bad error message. Unfortunately I couldn't find a good way of wrapping the error message from the resolver since it might not be an IOError with a NotFound kind, so it may not have an error message to wrap, and reaching into that felt a little too intrusive.

Refs #174.

[devongovett]

Thanks, this looks great!