Support for attributes on port connections

[mkurc-ant]

This PR adds support for defining attributes on port connections. Like that:

cell cell_instance (clk, rst, (* input_conn_attr = 10 *) inp, out);

I updated the RTLIL::SigSpec object so it holds a dict with attributes. Attributes are accepted by the Verilog and Ilang frontends. They are outputted by the Verilog, Ilang and JSON backends.

[mkurc-ant]

I already made some "performance" testing with and without this change. I ran Yosys with the 9-scalable-proc design with N=10. Each run was repeated 3 times to get rid of influence of random computer load.

YosysHQ/master:

CPU: user 6.94s system 0.05s, MEM: 221.55 MB total, 194.02 MB resident
Yosys 0.8+509 (git sha1 90ec2cda, gcc 7.4.0-1ubuntu1~18.04 -fPIC -Os)
Time spent: 21% 14x opt_merge (1 sec), 14% 7x techmap (1 sec), ...

CPU: user 6.97s system 0.04s, MEM: 221.55 MB total, 193.65 MB resident
Yosys 0.8+509 (git sha1 90ec2cda, gcc 7.4.0-1ubuntu1~18.04 -fPIC -Os)
Time spent: 21% 14x opt_merge (1 sec), 14% 7x techmap (1 sec), ...

CPU: user 6.92s system 0.07s, MEM: 221.55 MB total, 193.65 MB resident
Yosys 0.8+509 (git sha1 90ec2cda, gcc 7.4.0-1ubuntu1~18.04 -fPIC -Os)
Time spent: 21% 14x opt_merge (1 sec), 15% 7x techmap (1 sec), ...

This branch:

CPU: user 7.09s system 0.06s, MEM: 221.65 MB total, 193.83 MB resident
Yosys 0.8+498 (git sha1 1bdc7e9d, gcc 7.4.0-1ubuntu1~18.04 -fPIC -Os)
Time spent: 21% 14x opt_merge (1 sec), 15% 7x techmap (1 sec), ...

CPU: user 7.21s system 0.06s, MEM: 221.65 MB total, 194.07 MB resident
Yosys 0.8+498 (git sha1 1bdc7e9d, gcc 7.4.0-1ubuntu1~18.04 -fPIC -Os)
Time spent: 21% 14x opt_merge (1 sec), 14% 7x techmap (1 sec), ...

CPU: user 7.09s system 0.05s, MEM: 221.65 MB total, 194.14 MB resident
Yosys 0.8+498 (git sha1 1bdc7e9d, gcc 7.4.0-1ubuntu1~18.04 -fPIC -Os)
Time spent: 21% 14x opt_merge (1 sec), 15% 7x techmap (1 sec), ...

The memory usage is increased by ~100kB

[mithro]

LGTM but needs some tests to show it actually works?

[mkurc-ant]

Good, I'll work on adding those tests.