Previously, `showAstData` produced a `String`. That `String` would
then be converted to a `Doc` using `text` to implement
`-ddump-parsed-ast`. But rendering `text` calculates the length
of the `String` before doing anything else. Since the AST can be
very large, this was bad: the whole dump string (potentially hundreds
of millions of `Char`s) was accumulated in memory.

Now, `showAstData` produces a `Doc` directly, which seems to work
a lot better. As an extra bonus, the code is simpler and cleaner.
The formatting has changed a bit, as the previous ad hoc approach
didn't really match the pretty printer too well. If someone cares
enough to request adjustments, we can surely make them.

Reviewers: austin, bgamari, mpickering, alanz

Reviewed By: bgamari

Subscribers: mpickering, rwbarton, thomie

GHC Trac Issues: #14161

Differential Revision: https://phabricator.haskell.org/D3894