Assertion operators

[h2non]

• [x] equal
• [ ] deep.equal
• [x] strict.equal
• [x] be
• [x] be.true
• [x] be.false
• [x] be.none
• [x] be.a / be.an / be.type
• [x] be.empty
• [x] be_above
• [x] be.below
• [x] be.above_or_equal
• [x] be.below_or_equal
• [x] be.within
• [ ] approximately
• [x] be.callable
• [x] have.len / have.length
• [x] have.property
• [x] have.properties
• [x] have.key
• [x] have.keys
• [ ] have.value
• [ ] have.values
• [x] contain
• [ ] contain.exactly
• [ ] contain.only
• [ ] contain.deep
• [x] startswith / start_with / starts_with
• [x] endswith / end_with / ends_with
• [x] match / matches
• [ ] match.each
• [ ] match.any
• [x] raise_error / raises
• [x] implements / satisfies
• [ ] first
• [ ] last
• [ ] head(size)
• [ ] tail(size)