for pattern mode, want a malloc implementation with no headers

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on April 29, 2012 12:08:53_

split from issue #794 the natural malloc replacement when using shadow memory places its header
in the redzone itself. this negatively affects pattern mode, however.

what pattern mode wants is a malloc implementation that uses no headers
whatsoever and uses a hashtable to store per-chunk data.
note that tcmalloc-type tricks don't really help b/c we need per-chunk
data like callstack and alloc type and not just size.
also note that a hashtable provides guaranteed invalid free detection,
which headers do not.

could go even further and chain frees in hashtable to have entire free
chunk filled w/ pattern

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=879_