Merge changes I8f33830f,Icd2b891b,I8736ff5c,Iae6e342a

* changes:
  libunwindstack: add method to iterate across registers.
  libbacktrace: expose BacktraceMap's unwindstack::Memory.
  libunwindstack: expose UnwindStackMap::GetFunctionName.
  libbacktrace: expose libunwindstack Unwind routine.