Calculate Proc#arity the same way as Method#arity

Proc#arity should return -(n+1) where n is the number of required arguments.
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
Merge pull request #815 from scooter-dangle/Proc#arity-with-default-args

Calculate Proc#arity the same way as Method#arity