BUG: allow Timestamp comparisons on the left
closes #4982 

whoosh you went all in on this! fused types!
 yeah ... there's an annoying vbench hit tho with index joins ... trying to find the source of it ... i actually got rid of the fused types, not necessary since really just `int`, `datetime` and `Series` are th possibilities
 ahh...ok
 @jreback comments?
 otherwise looks ok
 cool, i'll merge after next travis pass...
