API for hooking typed Swift functions

[maltek]

We want a wrapper around Interceptor.attach that's given the type signature of the hooked function and provides the installed hooks with the nice high-level proxies from #1 for parameters and return value. In other words, we need to implement reading parameters and return values according to the Swift calling convention for all the different kinds of types.

Maybe it'll become possible later to find the types of (some) functions automatically, but we'll still need this API as a building block.