Don't initialize the channels until we know how big they need to be - #665

[ucbjrl]

NOTE - This requires a new init_channels() method which must be called after init_sim_data(). Use symbolic constants in Tester.scala and sim_api.h for the channel data offset. Run TesterTests with both C++ and Verilog backends (if the latter is available). Add a test for dynamically sized channel code.

Update vcs support for big IOs.

[da-steve101]

Nice! fixed #665 Thanks