Resolve fully-qualified associated types

[orangeng]

In the parsing stage, the functionality of the syn::Impl parsing is extended to keep track of associated type implementations in trait Impls.

This info is kept in a hash-map, where the key is a combination of (self type, trait name, associated type name) and the value is the concrete type itself.

In the library/transformation stage, this populated hashmap is then referenced to resolve associated types found.

Closes #106

[mversic]

@emilio can we move this to completion? it's a great feature? @orangeng can you fix the lints?

[mversic]

@emilio is there something blocking the merge of this feature?

[mversic]

@emilio do you think we can get this PR merged soon?

[stegaBOB]

We're currently using this in prod. Would love to get this merged in! Without this PR out entire cbindgen flow wouldn't work. Thanks @orangeng for your awesome contribution!