Introduce <<< operator for new aggregation support

This patch introduce new aggregate operator: <<<
For example:
	s = ptable()
	s["test"] <<< 1
	s["test"] <<< 2
	print(s["test"])    ---> [count: 2 sum: 3 max: 2 min: 1 avg: 1]
	print(count(s["test"])		---> 2
	print(sum(s["test"])		---> 3
	print(max(s["test"])		---> 2
	print(min(s["test"])		---> 1
	print(avg(s["test"])		---> 1

This new aggregation mechanism is borrowed from systemtap,
also old ktap aggregation is killed now.

The reason for this change is new aggregation syntax will be more
easily for end user, "<<<" just look like stream operator, similar
with aggregation in some level.

In implementation, count/sum/max/min value stores in structure
ktap_stat_data, this structure is bind with table tightly, it allocate
with table nodes and array, and resize at same time. This will
avoid much ktap_stat_data memory allocation.

old builtin function "aggr_table" is replaced by "ptable",
which respesent parallel table.

The testcase(test/ptable.kp) and sample scripts are also updated.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>