EPA: encode keywords directly in AST

I voiced my concerns about the design of in-tree exactprint annotations here: !2418 (comment 338403). The result of the discussion was that we’d go with my proposed design, but implement it post-merge. This ticket is to track the proposed refactoring.

The new design is described as “Plan B” at https://gitlab.haskell.org/ghc/ghc/-/wikis/api-annotations. The idea is to introduce a new data type, HsToken, defined as follows:

type LHsToken tok p = XRec p (HsToken tok)

data HsToken (tok :: Symbol) = HsTok

Then we record token information directly in the syntax tree:

  data HsExpr p
    = ...
      ...
    | HsLet       (XLet p)
+                !(LHsToken "let")
                  (LHsLocalBinds p)
+                !(LHsToken "in")
                  (LHsExpr  p)

---

While working on this, I found the following commands useful:

hadrian/build -j --flavour=Quick --freeze1 test --test-root-dirs=testsuite/tests/printer
hadrian/build -j --flavour=Quick --freeze1 test --test-root-dirs=testsuite/tests/ghc-api/exactprint