Make Expectation constructor public

[mrcljx]

Some libraries (like RxNimble) create their own expectations. The following excerpt from RxNimble looks like a workaround to the lack of a public constructor:

extension Expectation {
    init(_ expression: Expression<T>) {
        self.expression = expression
    }

    // ...
}

But in current Swift versions this does not work anymore:

Initializer for struct 'Expectation' must use "self.init(...)" or "self = ..." because it is not in module 'Nimble'

That of course is not possible as the constructor is not public.

---

Not sure if this should be a patch or minor version bump.

[mrcljx]

This stopped working in Swift 4.1 because of SE-0189: Restrict Cross-module Struct Initializers.

[ashfurrow]

Makes sense to me! Will wait for another member of the team to merge.

[mrcljx]

Dropped the checkboxes from the PR description so that it looks more mergeable from the list.

[mrcljx]

@ashfurrow Is there anyone I could ping on this?

[ashfurrow]

@Quick/core This looks good to go, pretty small, additive change. Any objections to merging?

[ikesyo]

Makes sense 👍

[c0diq]

Could we get a new release to get this fix please?

[ikesyo]

Please follow #575.