Add the SQLITE_RESULT_SUBTYPE flag for application-defined functions. Add

Add the SQLITE_RESULT_SUBTYPE flag for application-defined functions.  Add
the -DSQLITE_STRICT_SUBTYPE=1 compile-time option that raises an error if
any function invokes sqlite3_result_subtype() without the SQLITE_RESULT_SUBTYPE
flag.  SQLITE_RESULT_SUBTYPE prevents an indexed value of that function from
being used to replace an equivalent expression, since the indexed expression
does not carry the subtype.

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