Better error message when using Python `TypeVar`

[mark-koch]

Type variables need to be declared via guppy.type_var(module, "T"). We should point users to this when they try to use a vanilla Python TypeVar instead.

See https://github.com/CQCL/guppylang/pull/210#discussion_r1598357444

This requires holding on to the Python scope in more places. In particular, we need to extract the Python scope when defining a struct/enum.