Solc should have an option to download a version and return it as a string

It should also do proper version verification and processing prior to downloading from solc-bin.

This was raised by @kobigurk, who wants to cache the downloaded versions.

Start by tracing the existing solc-bin download path and its version-handling entry points. Determine how an option could return the downloaded version as a string, then define and verify the expected behavior for version processing before download; done means the requested version is validated and the cached-download use case is supported.

Written by the indexing model from the issue text.