Various updates to support Guppy.

This adds the start of some pre-processor support, plus a new
group for "misc" language entities (e.g. "@comment ...").
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Assorted updates for more Guppy parsing and scoping;  test_g15.gpp now
goes through the front-end up to 'scope' successfully.
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Got test_g15 through type-checker, looks sensible.
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More concrete initialiser handling (incomplete).
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More updates for Guppy;  added KNOWNSIZEOF langop.