Support for embedded_alloc

[radius-jfluent]

It would be nice to be able to use embedded_alloc in a program using cortex_m_rtic. Right now, attempting this will result in a panic when the HEAP.init() fn is run (I'm assuming because it requires that it can utilize a critical section during the method). Maybe I'm missing something, but it doesn't seem like I can get it to work inside init() or a task.