Added the MemData attribute.
DRAM timing: Changed the default sentinel value to -1; made tRPab default to tRP.
Replaced the old hack for tick and InPort callbacks with std::function. Unfortunately this is about 5% slower, so we'll try to fix that.
Replaced the use of std::function with our own invocation wrappers, inexplicably *gaining* performance over the old non-method hack.