python - be: Implement text's sequence interface.

Log Message:
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Implement text's sequence interface.

Use text_substring and textlen :)

*_contains for text and bytea still needs to be implemented :(

Modified Files:
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be/src/type:
textual.c (r1.3 -> r1.4)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/type/textual.c.diff?r1=1.3&r2=1.4)