Merge pull request #1359 from YosysHQ/xc7dsp

DSP inference for Xilinx (improved for ice40, initial support for ecp5)

diff --cc backends/aiger/xaiger.cc
Simple merge

diff --cc techlibs/xilinx/abc_xc7.box
Simple merge

diff --cc techlibs/xilinx/synth_xilinx.cc
Simple merge