This makes the syntax for defining a memory map much less verbose.

Old:

let memory = BranchMemory::new()
  .map(0x0000, Box::new(ram))
  .map(0x4000, Box::new(io))
  .map(0x8000, Box::new(rom));

New:

let memory = BranchMemory::new()
  .map(0x0000, ram)
  .map(0x4000, io)
  .map(0x8000, rom);

This applies to Banked and Branch memory types.

This is accomplished by using impl syntax, replacing memory: Box<dyn Memory> with memory: impl Memory + 'static and constructing the Box within the .bank and .map functions. The 'static is needed to ensure the Box's contents live long enough.