Find a way to allow usage of Matcher class within custom matcher implementations.

[paweldudek]

Right now custom matchers might require complicated logic as they cannot leverage internal components of Mimus.

For instance, consider following scenario. We're trying to write a custom matcher for URLRequest. In that matcher we need to compare URL? and [String: String]?.

That means we need to do the usual dance of unwrapping, checking for equality etc:

public func equalTo(other: MockEquatable?) -> Bool {
    if let otherURLRequest = other as? URLRequest {

        var urlsMatch = false
        var headersMatch = false

        if self.url == nil, otherURLRequest.url == nil {
            urlsMatch = true
        } else {
            if let actualURL = otherURLRequest.url, let expectedURL = self.url {
                urlsMatch = actualURL == expectedURL
            }
        }

        if self.allHTTPHeaderFields == nil, otherURLRequest.allHTTPHeaderFields == nil {
            headersMatch = true
        } else {
            if let actualHeaders = otherURLRequest.allHTTPHeaderFields, let expectedHeaders = self.allHTTPHeaderFields {
                headersMatch = actualHeaders == expectedHeaders
            }
        }

        //TODO: Move this to Mimus? Find a way we can leverage Mimus.Matcher
        return urlsMatch && headersMatch
    }

    return false
}

This could be greatly simplified by using Mimus Matcher class:

public func equalTo(other: MockEquatable?) -> Bool {
    if let otherURLRequest = other as? URLRequest {
        let matcher: Mimus.Matcher = // Get the matcher from somewhere?

        return matcher.match(expected: [self.url, self.allHTTPHeaderFields], actual: [otherURLRequest.url, otherURLRequest.allHTTPHeaderFields])
    }

    return false
}