Implement len(chan) in frontend

Also, minor cleanup for evaluating builtin functions
Fixes to match go/types

- There is now a common object for function parameters,
  so we can ditch a bunch of code from check.go that
  post-processed the output of go/types.
. Final parameter type for variadic functions is now
  a slice type, so we can simplify some code that no
  longer cares what the type is (e.g. typemap.go)