Use Writer monad instead of State to annotate comments

[msakai]

resolves #22

[msakai]

The behavior appears to change from “replace” to “append,” but in reality, there is no difference.

The only place withComments is used is

logic-TPTP/ParserC.y

Lines 148 to 150 in b02af74

	quantified_formula :: {FormulaC}
	quantified_formula : quantifier lbra variable_list rbra colon unitary_formula
	{ $1 $3 $6 `withComments` comm $5 }

and the quantifier is defined as

logic-TPTP/ParserC.y

Lines 188 to 189 in b02af74

	quantifier :: {[V] -> FormulaC -> FormulaC}
	quantifier : exclam { for_all } | question { exists }

and for_all / exists are defined as:

logic-TPTP/Codec/TPTP/Base.hs

Lines 120 to 126 in b02af74

	-- | Universal quantification
	for_all :: Pointed c => [V] -> (F c) -> F c
	for_all vars x = (F . point) $ Quant All vars x
	-- | Existential quantification
	exists :: Pointed c => [V] -> (F c) -> F c
	exists vars x = (F . point) $ Quant Exists vars x

Therefore, mf in the above code is always point of something, i.e. the annotation is empty.