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

tests/arch/xilinx/fsm.ys

@@ -31,6 +31,7 @@ stat
 select -assert-count 1 t:BUFG
 select -assert-count 6 t:FDRE
 select -assert-count 1 t:LUT1
-select -assert-count 8 t:LUT4
+select -assert-max 1 t:LUT3
+select -assert-max 8 t:LUT4
 select -assert-count 5 t:MUXF5
-select -assert-none t:BUFG t:FDRE t:LUT1 t:LUT4 t:MUXF5 %% t:* %D
+select -assert-none t:BUFG t:FDRE t:LUT1 t:LUT3 t:LUT4 t:MUXF5 %% t:* %D

tests/techmap/zinit.ys

@@ -20,7 +20,8 @@ EOT
 equiv_opt -assert -multiclock zinit
 design -load postopt
 
-select -assert-count 20 t:$_NOT_
+select -assert-count 16 t:$_NOT_
+select -assert-count 4 t:$xor
 select -assert-count 1 w:unused a:init %i
 select -assert-count 1 w:Q a:init=13'bxxxx1xxxxxxxx %i
 select -assert-count 4 c:dff0 c:dff2 c:dff4 c:dff6 %% t:$_DFF_??1_ %i
@@ -52,7 +53,7 @@ design -load postopt
 
 select -assert-count 0 t:$_NOT_
 select -assert-count 1 w:unused a:init %i
-select -assert-count 1 w:Q a:init=13'bxxxx1xxxxxxxx %i
+select -assert-count 1 w:Q a:init=13'bx00x100000000 %i
 select -assert-count 4 c:dff0 c:dff2 c:dff4 c:dff6 %% t:$_DFF_??0_ %i
 select -assert-count 4 c:dff1 c:dff3 c:dff5 c:dff7 %% t:$_DFF_??1_ %i
 
@@ -142,7 +143,7 @@ EOT
 zinit
 
 select -assert-count 0 t:$_NOT_
-select -assert-count 0 w:Q a:init %i
+select -assert-count 1 w:Q a:init=24'b0 %i
 select -assert-count 4 c:dff0 c:dff2 c:dff4 c:dff6 %% t:$_DFFE_??0P_ %i
 select -assert-count 4 c:dff1 c:dff3 c:dff5 c:dff7 %% t:$_DFFE_??1P_ %i
 select -assert-count 4 c:dff8 c:dff10 c:dff12 c:dff14 %% t:$_SDFF_??0_ %i