feat: add IsError and NotIsError helpers

As Go's error wrapping has become more and more common, these types of
tests are becoming ubiquitous.

Fix byte array comparison

fix: upgrade repr

Upgrade hermit Go

feat: special-case diffing strings

fix: diff with identical input

fix: apply logic from last commit to NonZero

fix: treat zero length slices/arrays/maps as zero

Also run each test in a sub-test so we get more descriptive test output.

docs: update README

feat: allow EqualError to accept nil errors if the message is empty