change dtype promotion behavior for jit-invariance
Here are two desiderata for jax.numpy dtype promotion behavior:
1. follow what NumPy does
2. be invariant to `@jit`
The latter is much more important, so whenever the two are in tension we
prefer the latter. (Also we already can't do a perfect job following
what NumPy does, e.g. around its value-dependent dtype promotion logic.)
Issue #732 showed our code had a special behavior that essentially
handled a case of the former desideratum but also broke the latter. #732
also showed us (again) that our tests really should cover Python
scalars.
In summary, in this commit:
* revise jax.numpy dtype promotion behavior to be invariant to `@jit`
* add Python scalar types to lax_numpy tests
* simplify and update kron implementation to fix dtype issues
