Add memory heat map

[duncanamps]

Add heat map feature to count reads and writes to the different areas of memory. This should be a feature which can be enabled or disabled for performance. Typical benefits would be:

• See which elements of code get the most use and therefore the greatest beneficiaries from optimisation
• See if code is not being executed
• Monitor if program code is being overwritten by faulty software
• See if something is getting stuck in a loop and where it's happening
• Check how much of the stack is actually getting used in practice