Move interactive commands to the repl crate

[jmmv]

This is a follow up to #118. The std crate currently has various commands that are meant to be used from an interactive context only, like LOAD and SAVE, and it even has a concept of a InteractiveMachineBuilder. I sense that all of these belong in the repl crate.

One might argue that a program should be able to use LOAD, for example, to run a different program -- but that wouldn't work because, as soon as it did that, it'd lose control of what it is doing. So the answer here is that those programs should use RUN as described in #129. Need to review if there are other corner cases like that.