Change the sqlite3BtreeKeySize() interface into sqlite3BtreeIntegerKey() and

Change the sqlite3BtreeKeySize() interface into sqlite3BtreeIntegerKey() and
make it only work for table btrees.  Change sqlite3BtreeDataSize() into
sqlite3BtreePayloadSize() and make it work for all btrees.  Combine
sqlite3BtreeDataFetch() and sqlite3BtreeKeyFetch() into a single
sqlite3BtreePayloadFetch() routine.  These changes seem to make the
b-tree interface more rational and they reduce both binary size and
CPU usage.

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