Off-by-one tidyup.

ALLOC_AP, ALLOC_PAP and MKAP were all being constructed
with size arguments equal to (1+number of args/FVs) in
ByteCodeGen.schemeE, only for Interpreter.c to subtract 1
when fishing out the payloads. This commit drops the
up-and-downery.

Simplification spotted by Andy Moran