common: define kMiniHeapSize and use it in MeshableArena's CheapHeap

[bpowers]

It was surprising and frustrating to find a random 64 in MeshableArena's:

  CheapHeap<64, kArenaSize / kPageSize> _mhAllocator{};

is tied to the size of MiniHeap. Make that explicit by defining a constant, and ensuring the constant stays in line with the size of MiniHeap through the use of a static_assert.

Updates #104