Add the "VALUES-as-coroutine" optimization. Large VALUES clauses on an

Add the "VALUES-as-coroutine" optimization.  Large VALUES clauses on an
INSERT, for example, prepare and run in about half the time and with half
the memory.  This check-in also includes enhancements to the internal
sqlite3ExprIsConstant() routine to recognize pure SQL functions as constant
if they have constant arguments.

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