remove core/instructions/val in favor of core/value

[williamberman]

core/instructions/val is being replaced with the core/value module
some of the non abstract machine + tests were dependent on the core/instructions/val module so removed those as well

the contrib/tests/simple_lang.rs tests were dependent on core/instructions/val so I removed and replaced with contrib/tests/simple_langg.rs tests

also ran cargo fmt :P

[WilfredTA]

Thanks for making these changes. I was planning on making some of these only AFTER finishing the implementation of the new 'Value' module (specifically, concrete execution will interpret the terms in the AST of Vals, and symbolic execution will Transpile to the AST offered by z3).

With these changes, it will save a lot of time since this PR brings many pieces up to date with the new value module already.