* Fix spelling errors identified by gospel spelling linter.
* Fix lint issue in common/source_test.go identified by staticcheck.
* Fix vet issue in common/source_test.go idenitifiued by go vet.

This uses type aliasing to avoid the construction of a distinct type
name to expose the common.Source interface in cel, simplifying the API
docs and making it possible in future to share function literals between
packages.

This makes the example code rendered by pkg.go.dev appear as a small
main, improving UX.

Fix runtime cost ref.Val stack

* Fix benchmark tests
* Minor changes to type-testing utilities to improve performance

Make use of baseList functions to manipulate mutableList
Also provide fast path list concatenation when other list is a
*mutableList.

#347)

* Refactor the type substitution checks for simplicity and performance. This simple change roughly doubles check-time performance
* Add an option to early-validate declarations and to propagate validated declarations from one checker.Env to derivations created using Extend
* Benchmarks and tests for validated declaration copying
* Updates based on review feedback
* kind guard before proto equality check