Support of slice for Program code

[ibuki2003]

Now Program::code is ArrayVec type, which contains its own data. I think almost all of use cases are to program "static" one, and it is not necessary to allocate memory every time.

So I suggest to add slice support for Program::code.

Furthermore, I want pio_proc assembler macros to support slice as well. The macros returns ProgramWithDefines<> type, which has dynamic type parameter, so it is not possible to use it as static variable.

I am able to work with this change, but I am not sure if it is good idea or not.