strengthen semantics of CevalPrim1 for init global var

only allow updating a global var if it is NONE
start updating compile_val to allow global env to change

the old statement made no mention of the globals field of the bytecode
state (since it didn't exist), but we have to say something about it for
the compilation of extend/init gvar. i am hoping the relation i've
defined saying the global env grows monotonically is sufficient.