Add explicit support for type constructors

[electrum]

This adds an optional type constructor operator which can be used by
types that have a literal form that is different than casting from
varchar. Currently, the JSON type is the only such instance and
support for it was previously hard-coded in several places.

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[dain]

I no longer work on prestodb, and instead am focused full time on https://github.com/prestosql/presto

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you'd still like this PR merged, please comment on the task, make sure you've addressed reviewer comments, and rebase on the latest master. Thank you for your contributions!