Add simple MemBlock class for managing byte array allocation

A class to encapsulate self-deleting byte arrays while preserving access
to the underlying length (without the length being part of the type, e.g.
std::array<>). By design, the only interface to the underlying bytes is
via Slice, to encourage safer memory access usage.

Test: as follows
    - built
    - flashed
    - booted
    - make -j 128 netdutils_test && \
      adb push out/target/product/<foo>/data/nativetest64/netdutils_test/netdutils_test /data/nativetest64/netdutils_test && \
      adb shell /data/nativetest64/netdutils_test
      passes
Bug: 78250686
Change-Id: Id3f3a2c8af7ae443ecb4d67bf0326b031626f53b