Turn rhum into purely a mocking module

[ebebbington]

Summary

Because of how the testrunner for deno is changing, they offer 90% of what rhum does, meaning Rhum will soon become obsolete

It's also worth checking if theres already mocking libraries out there. If there is, can we do it better? if not, is it worth keeping rhum (sad face)?

[crookse]

this will be v2 of rhum. closing. there is an open PR: #132