time-bound memory-pool

O(1) realloc() for use with Lua.
A standard malloc/free/realloc API is exposed for testing and other
potential use-cases.

The current configuration it's performs well for lua-metatables
(regular calls to realloc() with varying tiny chunks ~1-50 bytes)
For the use-case at hand it outperforms TLSF.

libs/pbd/wscript

@@ -62,6 +62,7 @@ libpbd_sources = [
     'pool.cc',
     'property_list.cc',
     'pthread_utils.cc',
+    'reallocpool.cc',
     'receiver.cc',
     'resource.cc',
     'search_path.cc',