Parse enum definitions

[mark-koch]

Add a decorator @guppy.enum that parses a python class into a EnumDef.

Come up with syntax for struct enum variants. Maybe something like this:

@guppy.enum
class MyEnum:
    Variant1 = {
        "a": int,
        "b": Qubit
    }
    Variant2 = {}
    ...

Blocked by #160

[ss2165]

Could we have anonymous variants,

Variant3 = (int, float)

?

[croyzor]

Notes:

• If one variant of an enum contains a linear field, the whole enum becomes linear
• We should probably unify enums and structs as some ADT type internally