add Stop instruction to bytecode

this is a dummy instruction. when it is reached, that means the code
generated for a decelaration has finished executing (whether it succeeds
or fails).
make compile_dec deal properly with the possibility of an exception

following the scheme that if the generated code raises an exception, we
reset the stack and put a 1 on top (above the exception), otherwise we
update the stack with the new bindings and put a 0 on top.

compiler (emitml) tests and repl_fun (eval) tests still work.
compiler proofs broken.
fix bytecodeExtraTheory for changes to Push/PopExc
fix compileLabelsTheory for changes to Push/PopExc