More expectations

[ilyapuchka]

• beNotNil to validate that value is not nil
• notThrow to validate that function does not throw
• throw(_ match: (Error) -> Bool) to validate that function throws error using custom condition rather than Equatable
• throw and throw(_:) expectations to better match other APIs (where to is omitted)
• expect(...).to.not for not expectations, expect(...).to has now better scoped methods and code completion

[kylef]

Adding a not expectation type was on my list of things to implement for some time. I was thinking of adding a not "keyword" on an expectation so you could chain not to various types. I've made a start in #33. What do you think?

[ilyapuchka]

@kylef that's nice! Though I find it a bit confusing that things like try expect("kyle").to.to.not.to.to.to.beNil() are possible. Maybe we can use a phantom type or a separate expectation type to allow only specific methods after to or not? Maybe also not.to or just notTo would be better than to.not?