Resource objects are no longer required to implement __hash__.

Each memory map now has a namespace for its resources and windows.
Names are mandatory for resources, but optional for windows. The full
name of a resource is a concatenation of its own name and the name of
each named window behind which it is located.

Breaking changes:
* MemoryMap.add_resource() now takes a mandatory ``name`` parameter.
* MemoryMap.resources() now returns a ``resource, name, (start, end)``
  tuple.
* Freezing a memory map now prevents the further addition of *any*
  resource or window, instead of only the ones that require an extension
  of the memory map address bits. Allowing these could prevent us from
  detecting name conflicts in some cases.

Other changes:
* The MemoryMap constructor now takes an optional ``name`` parameter,
  which is then visible as a property.
* The constructors of csr.Multiplexer, csr.Decoder, WishboneCSRBridge
  and wishbone.Decoder now take an optional ``name`` parameter, which is
  passed to their underlying memory map.

Breaking changes:
* MemoryMap.all_resources() and MemoryMap.find_resource() return an
  instance of ResourceInfo describing the resource and its assigned name
  and address range, instead of a tuple.

Fixes amaranth-lang#20.