improve version checks

Improve the Go version check for range-over-int and range-over-func.
Use the go/version package instead of custom parsing.

Also, skip the test of the new range features until we understand
the behavior better. They seem to fail on Go 1.23 (see
https://go.dev/play/p/O_l9BVAhGGu).

improve checked templates doc

make len check conservative for interface types

improve coverage

split builtin checks to a separate file

Pure code motion.

update doc to remove funcmaps discussion

finish test coverage

add doc