add ffi printk example into README

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
delete samples/ffi/halt.kp

This ffi script is too dangerous, should not to be sample.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
rename interpreter directory to runtime

"interpreter" naming is not properly anymore, it make
people think it's a slow language interpreter, there also
have possibility ktap will use some JIT in future, and
that directory include more files not connected with
normal interpreter, so use "runtime" would be more properly.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
rename test/bench to test/benchmark

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