code cleanup for if-const dead code removal: move handling into ConstantFolding transform, where it can also replace the entire node

--HG--
rename : tests/run/consts.pyx => tests/run/if_const.pyx

- Silent compiler warnings about unused function/parametes
- Defined to __attribute__((__unused__)) for GCC(>3.4) and ICC
- Applied to a __{get|release}buffer__ special methods
- Applied to a couple of (self,unused) parameter pairs