Try overwriting supervisor stack to reclaim RAM

[projectgus]

This was brought up in #76 but the initial fix for the issue doesn't address it, so I'm spinning it off here.

When the RTOS scheduler starts we reserve the memory used for the pre-scheduler stack as "supervisor stack", but if scheduler never exits then we can probably clobber this memory and reuse it for heap.

Amount to potentially save is unclear. sdk_user_start has the stack at approx 0x3ffffce9, and the current high water mark when the scheduler starts is 0x3ffffc10. So potential saving may be ~217 bytes, or may be more if the ROM functions also allocate stack memory before calling user_start that can be reused.

Will need to tread carefully as some ROM functions may also make use of hard-coded memory regions at the top of memory here.

The task for someone to try is:

• When the scheduler starts, set the stack limit to 0x3ffffce9 (or higher) and see if any memory corruption issues appear.
• Devise automated tests to try and expose any likely memory corruption issues (context switches, calling ROM functions, etc.)