Refactor program generator

[theosotr]

This PR contains cumulative changes including but limited to

• Support for wildcard types and use-site variance
• Refactor program generator (now there's no need for transformations, e.g., type creation, etc.)

This addresses a couple of issues:

• Closes https://github.com/theosotr/check-type-system/issues/24
• Closes https://github.com/theosotr/check-type-system/issues/20
• Closes https://github.com/theosotr/check-type-system/issues/5
• Closes https://github.com/theosotr/check-type-system/issues/21

In the future, we need to remove the code related to these obsolete program transformations.

[theosotr]

@StefanosChaliasos ready