Implement multi-ported FPGA memories

[tilk]

To implement superscalarity on FPGAs, we need multi-ported FPGA memories. Such memories internally use two-ported FPGA memories and some glue logic to make them behave as a single multi-ported memory.

Here is a nice paper: Modular Multi-ported SRAM-based Memories.

Interface suggestion: the new memories should probably share the interface with amaranth.lib.memory as much as possible.