Rename INTERNAL_TIMER to PRIMFUN_TIMER for better clarity
Add BEGIN_RFUNC_TIMER/END_RFUNC_TIMER macros

Add seperate macros for timing R functions in eval.c so a group
enable/disable option can be implemented at a later time.
Initialize variables when calculating BuiltinSub/SpecialSum

(guess why I prefer compiling with -Wall -Werror...)