"call modes" of toAnonVarMutation

[jvoigtlaender]

See 0fb9d19#commitcomment-84354069.

Also, there is actually a

prolog-test-server/src/Server.hs

Lines 74 to 76 in a1b8f29

	S.post "/to-anon-var-mutation/indiv/:num" $ do
	num <- S.param "num"
	anonVarMutation (Indiviual num)

but no handling for it in

prolog-test-server/src/ToAnonymousVarMutation.hs

Line 15 in a1b8f29

toAnonVarMutation :: Mode -> ProgramText -> Either Error [ProgramText]