Add input types to chrono type casters

[tjstum]

The type casters in stl/chrono.h accept various types in their
from_python calls, but they currently just use a name for their
output type.

This uses io_name to differentiate, adding the various acceptable
types that the casters permit at runtime

This also includes type stub testing for the test_chrono_ext module. The
first commit in this PR contains the stubs only.

[tjstum]

I now understand why there aren't stubs for these functions. :-)

Is there a straightforward way to tell stubgen to emit the module attribute value as = ... in the stubs (instead of as = True or = False)? It seems like, in general, we would always want that (since the runtime value isn't relevant: just the type). I could make that change as a separate PR.

Failing that, I can reduce this PR to just the type caster change and then just leave this early version as "proof" of its correctness.

[tjstum]

I've reduced the PR to just the type caster change. Hopefully this is amenable!