Small optimisation, save a bit of memory from MicroBitIO.

[microbit-carlos]

Display and buttons pull configuration is done in their own components.
Moving from a ManageBuffer to a plain array saves another 100-ish bytes.
In total with arm-none-eabit-gcc v10.3 this PR saves 174 bytes of flash and moves out the heap allocation for the ManageBuffer.

[github-actions]

Build diff

Base commit: 78fc71323d231dce9c22287dbe62903f8e383f04
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/13418095941

     VM SIZE    
 -------------- 
  +0.2%     +12    /home/runner/work/codal-microbit-v2/codal-microbit-v2/source/main.cpp
  +0.0%      +2    [section .text]
  -0.0%     -32    [section .heap]
 -10.9%    -174    /home/runner/work/codal-microbit-v2/codal-microbit-v2/libraries/codal-microbit-v2/model/MicroBitIO.cpp
  -0.0%    -192    TOTAL