Add stack measurement to the STM32 framework

[bifurcation]

The cortex-m-rt crate supports a paint-stack feature, which adds code to the binary to paint the stack before the program runs. We should add a few things so that we can do stack usage measurements based on stack painting:

• Add code to find the "high water mark" of the paint after some program execution
• Add code to "repaint" the stack to allow incremental measurements

There is some initial code in ui-stm32/src/stack.rs for these functions, and ui-stm32/src/bin/stack.rs is a small test binary. Be cautious with these, however -- running an earlier version of the test binary rendered my UI chip completely non-functional.