proc.h: MRB_ASPEC_REST() macro return 1 or 0

MRB_ASPEC_REST() is macro just only checks whether truthy or falsy.
And truthy means exist 1 rest argument in current proc.
OP_ENTER: Rewrite split aspec use macros
Merge pull request #2422 from ksss/aspec-macro

OP_ENTER use MBR_ASPEC_* macros