Make Proc#arity distinguish between lambdas/procs

Commit 4a09527aada1967ed6ae09dec8d88d109851b5d4 neglected to make a
distinction between how Proc#arity worked for lambda's and for plain procs.
It caused (I think) 4 tests to fail.

This commit fixes that. Those 4-odd tests again pass and now 4 tests that
previously failed also pass leaving only 1 fail tag in proc/arity_tags.txt
Remove now-passing Proc#arity spec tags