Add support for custom failure messages in Spec expectations

[Fryguy]

This PR introduces a breaking change if people happened to be using file and line as positional parameters, which is why I'd like to get this in before 1.0. The reason I introduced this breaking change is because as a user I'd expect the failure message to come immediately after the expectation (as it is in RSpec), and I believe it will be much more likely for users to use custom error messages than file and line (based on my experience in Ruby with RSpec).

The fix for the file and line change is very straightforward. In this PR I've fixed the core specs by using keyword notation, since core had both keyword-style and positional-style, and keyword-style seemed cleaner. I had even considered "forcing" keywords to be used for the file and line parameters, but thought that might warrant some discussion first.

See also the brief discussion in this thread: https://forum.crystal-lang.org/t/spec-assertion-with-custom-message/1781

[straight-shoota]

I would make file and line named arguments, even without this change. With it, even more so.

[Fryguy]

@straight-shoota I agree (just to clarify, when you say named arguments, you mean the "forcing" of the caller to use keywords, right?)

[oprypin]

Probably should split the file,line named arguments thing into a separate PR, not to taint this one as breaking-change.

[straight-shoota]

@oprypin failure_message replaces file as second positional argument, so it's definitely a breaking change. And it can even break silently. Although that shouldn't be too much of a problem because typically both file and line are passed and then the argument types don't match. And it's also "just" spec output, so if it fails and prints a message instead of a file name, that's not a big deal. You can easily fix it.

[bcardiff]

Can we add the following declarations to get deprecation warnings instead of compile-time errors?

    @[Deprecated("Use named arguments .should expectation, file: file, line: line")]
    def should(expectation, file, line)
      should(expectation, nil, file: file, line: line)
    end

    @[Deprecated("Use named arguments .should_not expectation, file: file, line: line")]
    def should_not(expectation, file, line)
      should_not(expectation, nil, file: file, line: line)
    end

There are some shards that are using file, line for custom error messages and I would rather offer a transition here.