changes to per-thread cache algorithms

Core algorithm changes:

* Per-thread cache is refilled from existing fastbins and smallbins
  instead of always needing a bigger chunk.

* Caches are linked, and cache is cleaned up when the thread exits
  (incomplete for now, needed framework for chunk scanner).

* Fixes to mutex placement - needed to sync chunk headers across
  threads.

Enabling the per-thread cache (tcache) gives about a 20-30% speedup at
a 20-30% memory cost (due to fragmentation).  Still working on that :-)

Debugging helpers (temporary):

* __malloc_scan_chunks() calls back to the app for each chunk in each
  heap.

* _m_printf() helper for "safe" printing within malloc

* Lots of calls to the above, commented out, in case you need them.

* trace_run scans leftover chunks too.

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