Initial attempt at checks_ext

[jonasfj]

There is a bit of AI experimentation going on here.

I think more extensions for check() is a good thing, maybe some of the good ideas from here can one day land in package:checks -- this package is an attempt to be more opinionated in the direction that check(something) should be useful, for most cases of something.

Some open questions:

• (A) check(myCompleter).isCompleted() vs check(myCompleter.isCompleted).isTrue(), where is the line for what extensions should offer and what should be preprocessed?
• (B) Messages from check('a\nb\n\c').lines.contains('a') say has lines that:, but maybe the messages could be where lines ... or something else, because .lines is not really a property of String, but useful a derived value.
• (C) Sometimes, I find the error messages less than pretty, maybe that's a bug or maybe that's just the way they should be.
• (D) Maybe, I need better guidelines for:
  • When to add extensions and what methods/properties to add.
  • What words / terms to use in description messages.
  • When to use custom pretty printing in description messages.
• (E) So far this package has minimal external dependencies, but we could add dependency on some popular packages to add check-extensions for types that they offer.

CC @natebosch WDYT?