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.  Fix for the problem described at
[forum:/forumpost/68d284c86b082c3e|forum post 68d284c86b082c3e].

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