objarray: Implement array slice assignment.
This is rarely used feature which takes enough code to implement, so is controlled by MICROPY_PY_ARRAY_SLICE_ASSIGN config setting, default off. But otherwise it may be useful, as allows to update arbitrary-sized data buffers in-place. Slice is yet to implement, and actually, slice assignment implemented in such a way that RHS of assignment should be array of the exact same item typecode as LHS. CPython has it more relaxed, where RHS can be any sequence of compatible types (e.g. it's possible to assign list of int's to a bytearray slice). Overall, when all "slice write" features are implemented, it may cost ~1KB of code.