Allow lowering constants of ndt/xnd

[saulshanabrook]

Users should be able to add constant xnd/ndt types, like this:

t = ndtypes.ndt("float64")

@numba.njit
def hi():
    return t

• Add as dummy types ref
• replacing str in wrapper with these constant types

However, this might be more trouble than it's worth, not supported by numba (see). So instead we could add lower_constant to python or C types, and serialize them from Python. Maybe that is most straightforward.