Split Transformer into a pure class and ProjectIn

[thomasjm]

Makes this repo's Transformer class match the other three, so the plumbing around it can eventually be shared.

Transformer keeps the pure methods — transformPosition, untransformPosition, unproject. ProjectIn m takes project, handleDiff, handleDiffMulti.

A single class Transformer m a does not work: the pure methods never mention m, so they become ambiguous at every call site. A fundep a -> m fixes that but pins each transformer to one monad, ruling out a mixed stack. With the split, a transformer states how much monad it needs — Monad m for pure ones, which keeps them usable at Identity, MonadIO m for ones that shell out to a parser.

getParams is gone. After Params left position mapping its only caller was unproject, which can read what it needs off the transformer value; cpp and go's implementations were fabricating a constant. unproject is now a -> Doc -> Maybe Doc with a Nothing default.

Also drops a stray comment left in Transformer.hs by the previous PR.

Needs MultiParamTypeClasses. Note project @Foo becomes project @_ @Foo, since m is the first type argument now.