use a pool allocator for IR allocations during encoding

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on November 04, 2011 10:28:59_

for regular DR there are minimal allocations during bb building as most instrs have valid raw bytes. but for clients inserting tons of instrumentation, each high-level instr is encoded to a temp buffer to figure out its length. a pool allocator would be most efficient there. however, note that encoding time is the bottleneck, so this is unlikely to be a huge win.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=600_