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	Switch off derivable type class generation by default
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	Merge to STABLE

This commit switches off the generation of the support code for 'Derivable
Type Classes' by default.  Use -fgenerics to switch it on.

Motivation: seldom used, and there's a nasty blowup in Core types for data
types that have lots of constructors or lots of fields.

We now put -fgenerics in the GhcLibOpts, so that the libraries have the
support code, as before.