Turn the heap into an optional module

[salkinium]

This allows for a heapless mode, which asserts on malloc and new operators (or deletes them?) and removes the heap and its initialization code from the linkerscript.

This allows ultra-low memory devices (AVRs and STM32L0) to use more of their RAM for static data and to assure developers that they are indeed not using any heap.
Our HAL and most classes should support this out of the box, otherwise they can transparently depend on the heap module (which maybe logs that is is used).

[salkinium]

Oh, just discovered we already have a module called :architecture:heap, which currently only defines the additional memory traits.
But it's always depended upon and implemented by :platform:core/:platform:cortex-m, so that needs to be actually implemented properly.