Investigate adding heap canaries

[AntonioND]

Stack overflows in particular are very hard to detect and very dangerous, it would be very nice to have a system to create debug builds of a program that can detect this kind of issues.

The following can be used as inspiration: https://github.com/profi200/libn3ds/blob/3bd21685705cbcc4ada9aab2c0f43faeeb304151/source/arm11/debug.c#L309-L386

EDIT: We already have stack canaries thanks to @asiekierka, only heap canaries are left.

[AntonioND]

As I've said in blocksds/libnds#98, now that we have stack protector support in libnds, we need a test for the stack protector (in examples/debug).

We should also add this to the documentation, eventually, as well as the default exception handler.

[asiekierka]

There's a test in BlocksDS now, and a separate documentation issue is open: #154

This leaves heap canaries.