Decouple HELP from std

[jmmv]

The HELP command is purely a feature of the REPL so it should be in the repl crate, not std.

Moving the command should be easy to do, except maybe for its tests that rely on the std MachineTester.

The more interesting question is: what do we do about the descriptions attached to all commands and functions? Somehow it does not make sense for those to be in the std crate because they are a user-facing, but keeping those descriptions attached to the callable objects makes it easier to update them. If we moved everything to the repl crate, then we'd need a way to ensure all registered commands and functions are documented, which kinda violates layering principles...