Added exported imports for several Alamofire types.

[DenTelezhkin]

While TRON is a wrapper around Alamofire API's, several of Alamofire types have been exposed through TRON interface, forcing a user to explicitly write import Alamofire along with import TRON to use them.

It's possible to write an implicit import, that exports entire framework to another framework users(see example, advanced example). TRON - Alamofire situation is a little different, however, since TRON wraps some API's and discourages user from using Alamofire API's directly.

Therefore, this PR aims to export only specific types from Alamofire, that are actually exposed by TRON API.

Downside of this PR is that if TRON API user had local API's that clash with Alamofire naming(for example, local Session type, or local Request type), this change would actually be breaking and would require to explicitly disambiguate those types(Alamofire.Request).

[codecov]

Codecov Report

Merging #77 into master will not change coverage.
The diff coverage is n/a.

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