use cBOOL for bool casts

use cBOOL for bool casts

bool b = (bool)some_int

doesn't necessarily do what you think. In some builds, bool is defined as
char, and that cast's behaviour is thus undefined. So this line in mg.c:

    const bool was_temp = (bool)SvTEMP(sv);

was actually setting was_temp to false even when the SVs_TEMP flag was set.
Fix this by replacing all the (bool) casts with a new cBOOL() cast macro
that (hopefully) does the right thing.