FfData: some refactoring.

- FfData now keeps track of the module and underlying cell, if any (so
  calling emit on FfData created from a cell will replace the existing cell)
- FfData implementation is split off to its own .cc file for faster
  compilation
- the "flip FF data sense by inserting inverters in front and after"
  functionality that zinit uses is moved onto FfData class and beefed up
  to have dffsr support, to support more use cases

passes/memory/memory_dff.cc

@@ -581,7 +581,7 @@ struct MemoryDffWorker
 		// Now we're commited to merge it.
 		merger.mark_input_ff(bits);
 		// If the address FF has enable and/or sync reset, unmap it.
-		ff.unmap_ce_srst(module);
+		ff.unmap_ce_srst();
 		port.clk = ff.sig_clk;
 		port.en = State::S1;
 		port.addr = ff.sig_d;