* Correct slice indexing convention in code comment
A colon is not supported.
* Add more key slices tests
* Unify `has-key` tests with those from `pkg/eval/vals/has_key_test.go`
* Fix key slice format in documentation
Fixes#1646.
* Fix missing bracket
* Fix indexing
* Add more key slices `has-key` tests
* Fix `has-key` test expected value
Qualified imports of pkg/tt outnumber unqualified (27 to 24). Improve
consistency, and clarity, by changing the dot (unqualified) imports of
that package symbols to qualified.