Add ability to use a stub to overwrite a method

[oxinabox]

Right now stubs never overwrite methods.

But it would be quiet possible to have a:

something like overwrite(module, stub),
that would iterate through each signature a sub accepts
and call @eval(module, $stub.name(<ARGSSIG>)=stub(<ARGS>)

But I think this can wait until Cassette based Mocking.jl arrives; so as to ensure it will be compatible with that, rather than unrecoverably overwritting every method

[omus]

See JuliaTesting/Mocking.jl#53