Make Expectation constructor public
Make Expectation constructor public

Some libraries (like `RxNimble`) create their own expectations.

This [extension from RxNimble](https://github.com/RxSwiftCommunity/RxNimble/blob/602c14aae95db274004cc77ff1779a5feaf6f6fc/Source/Expectation%2BExt.swift#L4-L6) seems to have worked in earlier versions of Swift but not anymore:

> Initializer for struct 'Expectation<T>' must use "self.init(...)" or "self = ..." because it is not in module 'Nimble'
Quick/Nimble,twobitlabs/Nimble,twobitlabs/Nimble,twobitlabs/Nimble,Quick/Nimble,phatblat/Nimble,Quick/Nimble,AnthonyMDev/Nimble,AnthonyMDev/Nimble,phatblat/Nimble,AnthonyMDev/Nimble,phatblat/Nimble