drivertools: Utility code for indexing and traversing signal drivers

It adds `DriveBit`, `DriveChunk` and `DriveSpec` types which are similar
to `SigBit`, `SigChunk` and `SigSpec` but can also directly represent
cell ports, undriven bits and multiple drivers. For indexing an RTLIL
module and for querying signal drivers it comes with a `DriverMap` type
which is somewhat similar to a `SigMap` but is guaranteed to produce
signal drivers as returned representatives.

A `DriverMap` can also optionally preserve connections via intermediate
wires (e.g. querying the driver of a cell input port will return a
connected intermediate wire, querying the driver of that wire will
return the cell output port that's driving the wire).

Makefile

@@ -598,6 +598,7 @@ $(eval $(call add_include_file,kernel/celltypes.h))
 $(eval $(call add_include_file,kernel/consteval.h))
 $(eval $(call add_include_file,kernel/constids.inc))
 $(eval $(call add_include_file,kernel/cost.h))
+$(eval $(call add_include_file,kernel/drivertools.h))
 $(eval $(call add_include_file,kernel/ff.h))
 $(eval $(call add_include_file,kernel/ffinit.h))
 $(eval $(call add_include_file,kernel/ffmerge.h))
@@ -638,6 +639,7 @@ $(eval $(call add_include_file,backends/rtlil/rtlil_backend.h))
 OBJS += kernel/driver.o kernel/register.o kernel/rtlil.o kernel/log.o kernel/calc.o kernel/yosys.o
 OBJS += kernel/binding.o
 OBJS += kernel/cellaigs.o kernel/celledges.o kernel/cost.o kernel/satgen.o kernel/scopeinfo.o kernel/qcsat.o kernel/mem.o kernel/ffmerge.o kernel/ff.o kernel/yw.o kernel/json.o kernel/fmt.o
+OBJS += kernel/drivertools.o
 ifeq ($(ENABLE_ZLIB),1)
 OBJS += kernel/fstdata.o
 endif