tweak definition of whole_prog_compile

in particular, make it add the top-level exception handler even if
remove_labels is false. reorganise definition slightly.
add ml_string_to_labelled_bytecode to testLib

this function takes a string consisting of CakeML declarations (as they
would be typed at the REPL) and returns a list (as a HOL term) of
labelled bytecode instructions for running those declarations.