[Pp] replace every Format.formatter -> 'foo -> unit by 'foo printer

src/arith.mli

src/conflict.ml

src/conflict.mli

src/explanation.ml

src/explanation.mli

src/inputlang/altergo/symbols.mli

src/inputlang/altergo/why_ty.mli

src/inputlang/smtlib2/popop_of_smtlib2.mli

src/solver.ml

src/solver.mli

src/types.ml

src/types.mli

src/util/bag.mli

src/util/debug.ml

src/util/debug.mli

src/util/exn_printer.ml

src/util/exn_printer.mli

src/util/intmap.mli

src/util/intmap_hetero.ml

src/util/intmap_hetero.mli