thecl: fix remaining r/r conflicts and warnings

[DankRank]

this fixes the two final r/r conflicts mentioned in #64

I don't know if sub call as instruction and sub call as expression are really equivalent, so someone please test it.

I also fixed all of the warnings gcc gave me, so now you can finally build thtk without it giving you three screenfuls of "const qualifier discarded"

[Priw8]

This breaks some things, since there is a significant difference between sub calls as expressions and sub calls as instructions. The issues range from throwing incorrect errors to generating push instructions when they shouldn't be generated.

void noRet() {
    nop();
}

int testRet() {
    return 20;
}

void main() {
    noRet(); // throws "sub used in expression can not have void return type: noRet"
    int a = testRet(); // this one is fine: testRet sets the $I3 variable to 20, then $I3 is pushed to stack and assigned to a, as expected
    testRet(); // this one is NOT fine, since it also pushes $I3, which is not actually used anywhere later
}

Same kind of issues also happen with inline subs. This is something I mentioned here earlier.

[DankRank]

fixed it similarly to how i fixed instruction parameters