fix(types)!: do not try to infer types of overloaded functions

[mcous]

I had originally tried to support parameter and return type inference for overloaded functions in this library. TypeScript does not support this inference natively, so I used a (very cool) recursive type annotation to extract the different overload types.

This recursive utility, however, is prone to infinite recursion in various cases that I feel are more common that overloaded functions. In order to preserve a better experience for the general use case, this PR removes custom parameter and return type inference for overloaded functions.

This makes inference for non-overloaded functions much simpler and more reliable. For overloaded functions, there are now two options:

• TypeScript's native Parameter and ReturnType utilities will always infer the last overload; so if you're stubbing based on the last overload, you're good
• For other overloads, you can use the TFunc type parameter of when
  • e.g. when<(a: string) => string>(overloadedFn).calledWith('hello').thenReturn('world')