Added a new terminator for StgBacktrace

It's called rootBacktrace
Fixed offset problem with oFFSET_StgBacktraceHeader_bt

offsetBt is like oFFSET_StgBacktraceHeader_bt
but sensitive to weather or not profiling is turned on
Added assertion and added backtrace headers to interpreted heap objects
Save thunk backtrace to CurrentBacktrace when entering a thunk
Sanity check backtraces in heap

Sanity check to ensure that that all thunk and fun heap objects actually
have valid backtraces in their headers
Added depth field to backtrace closure