Add annotations for memory maps, wishbone and CSR primitives.

[jfng]

Depends on PR amaranth-lang/amaranth#978.

MemoryMap resources (e.g. csr.Elements) are now assumed to have a .signature property, which lets them hold annotations. I am not sure whether we should enforce this in MemoryMap.add_resource(), or introduce a base class instead.

[jfng]

Updated, mainly to track changes in amaranth-lang/amaranth#978:

• replace annotation names with their $id URLs;
• replace the Signature.annotations property with Signature.annotations(self, interface, /) to access memory maps, which are assigned to bus interfaces;
• improve URL paths of amaranth-soc schemas:
  • csr.Interface: from /csr.json to /csr/bus.json;
  • csr.Element: from /csr-element.json to /csr/element.json;
  • wishbone.Interface: from /wishbone.json to /wishbone/bus.json;
  • memory.MemoryMap: from /memory-map.json to /memory/memory-map.json.