kernel: Rewrite bufNormalize

This is a complete rewrite of the RTLIL-kernel-side bufnorm code. This
is done to support inout ports and undirected connections as well as to
allow removal of cells while in bufnorm mode.

This doesn't yet update the (experimental) `bufnorm` pass, so to
manually test the new kernel functionality, it is important to only use
`bufnorm -update` and `bufnorm -reset` which rely entirely on the kernel
functionality. Other modes of the `bufnorm` pass may still fail in the
presence of inout ports or undirected connections.

tests/various/aiger2.ys

@@ -157,6 +157,7 @@ splitnets -ports
 copy test gold
 flatten gold
 techmap submodule1
+opt_clean
 select test
 write_aiger2 -flatten aiger2_ops.aig
 select -clear
@@ -216,6 +217,7 @@ prep -top top
 techmap t:$add
 
 splitnets -ports top
+opt_clean
 write_aiger2 -flatten aiger2_flatten.aig
 flatten
 rename top gold