Thin-cell decoder changes.

Add hard decoder gates (nand, inv, pnan)
Add conditions for routing using LI layer in s8.
Generalize bus layers for decoders.
Move custom cells to own directory.
Fixed via directions, etc.
Add 4x16 hierarchical decoder and test case

compiler/bitcells/bitcell.py

@@ -7,7 +7,7 @@
 #
 import debug
 import utils
-from tech import GDS, layer, parameter
+from tech import GDS, layer
 from tech import cell_properties as props
 import bitcell_base