Imports are ignored

[JSAbrahams]

Description of Bug

The type checker currently ignores imports.
Everything in the project is in the global scope.

Expected behaviour

• Imports should be noted when creating the context.
• The type checker should analyse the signature of imported libraries.
• The type checker should throw an error if we try to use a function of a module that has not been imported (in the project)
• The type checker should use import aliases from .. import .. as <alias> where applicable