Add interface class member to Benchmark class

[maxmynter]

Add an Interface immutable dataclass to the Benchmark class that contains the signature information of a function. I.e.:

• Keys
• Values
• Key - Value Pairs
• Defaults

[nicholasjng]

That's a great test case!

Let's move Interface to the nnbench.types top-level though - we don't (presumably) need to use it anywhere except the Benchmark dataclass for now.

[nicholasjng]

Sorry I forgot, you also need to add interface to the Benchmark class as a field like so (otherwise it will not show up in the typing):

class Benchmark:
    """
    ...
    interface: Interface
        The benchmark function's interface, set in the post-init hook.
    """
    ...
    interface: Interface = field(init=False, repr=False)

[nicholasjng]

Nice job! You can merge directly after attaching the interface to the Benchmark.