Make allocator configurable

There are reports from increasing memory consumption over time that disappear when a different allocator is used. So the problem disappears when using memory tracking tools like https://github.com/iovisor/bcc/blob/master/tools/memleak.py

https://stackoverflow.com/questions/59008623/memory-leak-disappears-when-using-valgrind discusses cases where this can happen.

Terra reports that the use of tcmalloc or jemalloc avoids the problem.

It can be fixed with using [the jemallocator crate](https://crates.io/crates/jemallocator) as shown in https://github.com/CosmWasm/wasmvm/compare/v0.16.1...terra-money:release/v0.16.1.

It would be great to upstream that fix, at least as an op-in feature. Also we should better understand why it helps.