utils: not, and, or, sample, & empty

new primitive: literal

new util/arb: notEmptyString

had to refactor suchthat implementation into separate file so it could
be used internally.

rework/fix code to pass coverage thresholds

fix utils.not doc comment

update jsc.utils.sample w/ example

update README
Refactor PR
Merge pull request #45 from jsverify/btgerst-master

Btgerst master