WIP: hsSyn: add a new GhcSe phase for representing completely serialisable ASTs

Where by serialisable, we mean in the sense of GHC's Binary module.

This patch comes with utility functions for converting to/from GhcPs. The main difference with GhcPs ASTs is the representation of names and (syntax level) types in several places in the AST data types, which are turned into serialisable values when converting to GhcSe, just like a huge majority of AST constructs.

In addition to the conversion functions, this patch adds lots of Binary instances for AST data types that are parametrized on the phase, but only when the phase is instantiated to GhcSe.

This MR is based on ghc/ghc!1326 (closed) and adds another commit on top of it.

Note: there also is a haddock.diff file at the top of the tree, which is a haddock patch that must be applied for haddock to build properly with these changes. I will submit it to the haddock repo soon, but it's a bit unfortunate that the haddock patch depends on the GHC changes too, nothing will build properly until the haddock patch lands and the submodule is updated accordingly.