Support threaded heap profiles in pprof

    - Add a --thread N option to select profile for thread N (otherwise, all
      threads will be printed)
    - The $profile map now has a {threads} element that is a map from thread id to
      a profile that has the same format as the {profile} element
    - Refactor ReadHeapProfile into smaller components and use them to implement
      ReadThreadedHeapProfile
Fix sdallocx() assertion.

Refactor sdallocx() and nallocx() to share inallocx(), and fix an
sdallocx() assertion to check usize rather than size.
check in the configure script