The test for it is not run by the test suit, it does not compile because of
missing includes somewhere, and fixing compilation is not the end of the story.
The `visit_each` seems to be simply never worked, running the test fails with a
wrong result.
The only potential usage of `visit_each` is the Signals1/2 library, and I did
not find any bug report about that.
- Doc updates
- Fixed a minor bug in the creating scopes with lambda and let
- Speed up of the lambda and let compile times
- Made use of the new proto::basic_default_domain
[SVN r72178]
