Use Version objects for solc versions

[iamdefinitelyahuman]

Overview

Use Version objects to represent solc versions, instead of strings.

Specification

1. solcx/install.py - store the solc_version variable as a Version and update related code.
2. Return Version object(s) anywhere a version is returned to a user, e.g. get_available_solc_versions and get_installed_solc_versions
3. All functions receiving a version in the user input should be able to understand both strings and Version objects, e.g. set_solc_version, install_solc
4. Remove the get_solc_version_string method (this information is still available from get_solc_version).
5. Update unit tests to ensure all new/modified code is adequately covered.

References

• semantic_version docs: the Version class

Dependencies

This will amount to a breaking change, so cannot happen until 1.0.0.

[pegahcarter]

I got this