Resolve #700. Expose GToJSON.

[phadej]

Rename the type-class to be GToJSON' (with prime),
to avoid name clash with type-aliases GToJSON and GToEncoding

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This is the second of remaining 1.5 tasks.

[phadej]

EDIT: In fact, I'll be happy with the state for 1.5 to be released. #653 needs a bit of thinking, I won't have time for that soon. (And we have another major release on the horizon with scanner stuff). So let's not wait any longer. ping @bergmark

[bergmark]

Thanks!