Phew, this ended up being a pretty complicated change!
The basic summary is that we now support the syntax
```
@jaxtyped(typechecker=typechecker)
def f(...): ...
```
and when using this, we now get pretty error messages about what went
wrong.
(
The old syntax, i.e.
```
@jaxtyped
@typechecker
def f(...): ...
```
is still supported, but doesn't give much information.
)
The internals of this do quite a lot of magic! In particular we
dynamically create quite a lot of functions and test the provided
arguments against their signatures. The overhead should still be
minimal under `jax.jit`, though.
(TODO: what's the overhead like in non-jit situations, e.g. PyTorch?
I've tried to minimise the overhead throughout just to be sure, but
perhaps PyTorch users should stick to the old syntax?)
jianlijianli