This replaces the first-fit algorithm used in the Arena with a best-fit. According to "Dynamic Storage Allocation: A Survey and Critical Review", Wilson et. al. 1995, http://www.scs.stanford.edu/14wi-cs140/sched/readings/wilson.pdf, both startegies work well in practice.

The advantage of using best-fit is that we can switch the O(n) allocation to O(log(n)). Additionally, some previously O(log(n)) operations are now O(1) operations by using hash maps. The end effect is that the benchmark runs about 2.5 times faster on my machine:

    # Benchmark, evals, iterations, total, min, max, median
    old: BenchLockedPool, 5, 530, 5.25749, 0.00196938, 0.00199755, 0.00198172
    new: BenchLockedPool, 5, 1300, 5.11313, 0.000781493, 0.000793314, 0.00078606

I've run all unit tests and benchmarks, and increased the number of iterations so that BenchLockedPool takes about 5 seconds again.

Changes in #12048 cause a compilation error in Arena::walk() when
ARENA_DEBUG is defined. Specifically, Arena's chunks_free map was
changed to have a different value type.

Additionally, missing includes cause other compilation errors when
ARENA_DEBUG is defined.

Second commit fixes segfault in allocator_tests/arena_tests
The test uses reinterpret_cast<void*> on unallocated memory. Using this
memory in printchunk as char* causes a segfault, so have printchunk take
void* instead.

Reproduced with:

make CPPFLAGS=-DARENA_DEBUG

make check
test_runner.py
./bitcoin-qt -> settings -> Encrypt Wallet -> set password and/or change password if already encrypted
./bench_bitcoin

../configure CPPFLAGS=-DARENA_DEBUG
make check
test_runner.py
ABC_BUILD_NAME=build-asan build-configurations.sh