Summary:
This makes it possible to serialise Names and FastStrings in user
programs, for example, when writing a source plugin.

When writing my first source plugin, I wanted to serialise names but it
wasn't possible easily without exporting additional constructors. This
interface is sufficient and abstracts nicely over the symbol table and
dictionary.

Reviewers: alpmestan, bgamari

Reviewed By: alpmestan

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15223

Differential Revision: https://phabricator.haskell.org/D4782