Prefix operator for text and spgist support

Hi,

Attached patch introduces prefix operator ^@ for text type. For 'a ^@ b'
it returns true if 'a' starts with 'b'. Also there is spgist index
support for this operator.

It could be useful as an alternative for LIKE for 'something%'
templates. Or even used in LIKE queries instead of ~>=~ and ~<~ in the
future. But it would require new strategy for btree.

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Ildus Kurbangaliev
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company