Support window function API about plv8 HOT 2 CLOSED

Here's the thought.

plv8 window function will defines `winobj` object in local scope.  It 
represents the variable with the same name in C API.  

{{{
winobj.get_current_position();
winobj.get_partition_row_count();
winobj.set_mark_position(pos);
winobj.rows_are_peers(pos1, pos2);
winobj.get_arg_in_partition(argno, relpos, seektype, set_mark);
winobj.get_arg_in_frame(argno, relpos, seektype, set_mark);
winobj.get_arg_current(argno);
}}}

get_arg_xxx are a bit ugly; I don't want to make function specify argument 
number.  Maybe we should come up with more user-friendly analogy.  Also I won't 
expose WinGetPartitionLocalMemory(), as it is too low-level, and winobj should 
probably handle that internally.

=== Appendix: Window function API in postgres ===
{{{
extern void *WinGetPartitionLocalMemory(WindowObject winobj, Size sz);

extern int64 WinGetCurrentPosition(WindowObject winobj);
extern int64 WinGetPartitionRowCount(WindowObject winobj);

extern void WinSetMarkPosition(WindowObject winobj, int64 markpos);

extern bool WinRowsArePeers(WindowObject winobj, int64 pos1, int64 pos2);

extern Datum WinGetFuncArgInPartition(WindowObject winobj, int argno,
                         int relpos, int seektype, bool set_mark,
                         bool *isnull, bool *isout);

extern Datum WinGetFuncArgInFrame(WindowObject winobj, int argno,
                     int relpos, int seektype, bool set_mark,
                     bool *isnull, bool *isout);

extern Datum WinGetFuncArgCurrent(WindowObject winobj, int argno,
                     bool *isnull);
}}}

db84508