kill e.allfield

event field string is static, no need to use e.allfield to
get, read format file in events/ directly.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
support get string type field of event

It means you can get prev_comm and next_comm field of
event sched:sched_switch

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
add scripts/sched_transition.kp

Example result:

[root@jovi ktap]# ./ktap -C 0 scripts/sched_transition.kp
Press Control-C to stop.
swapper/0 ... rcu_preempt ... rcu_sched ... swapper/0 ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1 ... sshd ... ktap ... kworker/0:1

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