Make the Vyper language test suite consumable

[xrchz]

The language tests (in tests/functional) are currently just written as Python code using pytest. This issue is to separate the test data out so that it can be used by others (notably by vyper-hol and ivy), and refactor the existing tests to also consume this data.

I'm pretty open to whatever test data format is easy to work with. For example, we could use JSON. The kinds of fields I would imagine to appear for each test record would include:

• test name
• sources: dictionary of named contract code strings
• sequence of calls (contract, function name, arguments) and expected outputs (return data)
• possibly: whether call arguments are supposed to be randomised, whether calls are supposed to revert

(I don't know whether this covers all the kinds of things you're already testing.)