Fix RandAddSeedPerfMon and move large stack objects to heap

Rebased version of #4236 by @arowser, changed to use begin_ptr from #4293. Also use vXXXX variable name syntax for the vectors...

**Testing:** I'm a bit wary to merge as it affects entropy collection for randomness. I'd like someone with windows to verify that this does the right thing (so, start with `-debug=rand` and look for the `RandAddSeed() XXX bytes` message).

Fine with me...