inspect.Signature is slow.

[seandstewart]

Currently, the largest bottle-neck for coercing callable inputs is the inspect.Signature object and binding inputs to the function args, which we need to do manually so that we can coerce to the appropriate type.

In my testing, running a simple function with one or two args is up to 100x slower as a result of using that object.

[seandstewart]

A custom implementation of bind is about 20%-30% faster. I've also added coercing at time of binding, so we're saving there.

For now, I'm not going to re-implement inspect.signature as a review of the current implementation didn't reveal any low-hanging fruit for improving performance. We'll instead rely on util.cached_signature for now.