should SinglePageStack use a vec or similar to provide an explicit structure for a stack of immutable SinglePageAllocation?

[thedavidmeister]

i just had a quick look through this but IIRC:

• SinglePageAllocation has public fields rather than getters, so can be mutated after creation
• SinglePageStack only has top, which tracks the position of the next allocation/deallocation as a u16
• keeping track of all SPAs in a stack and that they are FILO isn't handled by the stack itself

would it be helpful to put a Vec<SinglePageAllocation> in the SPS and make the SPA fields private?