Created by: Superjomn

This PR refactors the compile system of operator and kernel.

I add two CMake functions add_operator and add_kernel to make the registering more intuitive.

add_operator(fc basic ...)
add_kernel(fc_compute ARM basic ...)

A compile level is introduced, the CMake flag LITE_BUILD_EXTRA controls whether to compile with extra operators and kernels.

By default, the basic level operators and kernels are published.