Add a rCurrentBacktrace register
Add pushBacktrace function

pushBacktrace(Capability*, Tracepoint*)
  1. allocates a new current StgBacktrace heap object
  2. At its head is the Tracepoint passed into this function as
  argument.
  3. At its tail is the old current backtrace

This is rather preliminary functionality which will allow us to test
things out. It will require major changes later
Refactored Backtrace RTS headers.

*Split into Backtrace Types and Backtrace Prototypes. This will allow
 prototypes in backtrace/Prototypes.h to reference both the Tracepoint
 type and the StgBacktrace type.
Add fprintBacktrace function

*Add fprintBacktrace function to dump a backtrace to screen
*Have this function called on exception