Introduce ManagedMemory struct to simplify allocator usage

[FerdinandSpitzschnueffler]

Brief feature description

Using the BumpAllocator usually follows the pattern

uint8_t memory[MEMORY_SIZE];                          // create some memory
BumpAllocator memoryAllocator{memory, MEMORY_SIZE};   // use the allocator

The same is likely to be true for future allocators. This could be combined in a struct like

template<uint64_t MEMORY_SIZE>
class ManagedMemory {
  public:
    ManagedMemory() : allocator{&this->memory, MEMORY_SIZE} {}
    BumpAllocator& allocator();

  private:
    uint8_t memory[MEMORY_SIZE];
    BumpAllocator allocator;
}