Auto merge of #802 - RalfJung:machine, r=RalfJung

some lib.rs refactoring

Taken out of https://github.com/rust-lang/miri/pull/799 so that we can land it now and resolve merge conflicts instead of dragging them along.

Splits lib.rs into machine.rs for the machine state and trait impl, and eval.rs for the main evaluator loop and setting up the initial stack frame.