pgsql: In ALTER COLUMN TYPE, strip any implicit coercion operations

Log Message:
-----------
In ALTER COLUMN TYPE, strip any implicit coercion operations appearing
at the top level of the column's old default expression before adding
an implicit coercion to the new column type.  This seems to satisfy the
principle of least surprise, as per discussion of bug #1290.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        alter_table.sgml (r1.73 -> r1.74)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_table.sgml.diff?r1=1.73&r2=1.74)
    pgsql/src/backend/commands:
        tablecmds.c (r1.136 -> r1.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.136&r2=1.137)
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.182 -> r1.183)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c.diff?r1=1.182&r2=1.183)
    pgsql/src/include/optimizer:
        clauses.h (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/clauses.h.diff?r1=1.75&r2=1.76)