hy-repr probably shouldn't use the registered function of a supertype

[Kodiologist]

If you make a new class C that inherits from a class B with a registered hy-repr function f-B, but don't register a function for C, then hy-repr on an instance of C will call f-B, which can be misleading: it looks like you have an instance of B when actually you have an instance of C. We should probably fall back on repr instead.

For example, pandas's Timestamp class inherits from datetime, so hy-repr on a timestamp will give you the impression that you have a plain datetime.

Removing this feature will mean that hy-repr on a namedtuple will no longer just work, but that's probably an okay trade-off.