python - be: Improve bytea accessors.

Log Message:
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Improve bytea accessors.

Primarily, do not assume the Datum is not toasted. Use byteaoctetlen for length
and DatumGetByteaPSlice() for slicing.

(str() should probably return a raw string of the bytea instead of the typoutput
as slicing now returns byteas instead Python StringType. do later, tho)

Modified Files:
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    be/src/type:
        bitwise.c (r1.6 -> r1.7)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/type/bitwise.c.diff?r1=1.6&r2=1.7)