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
Backport of Core PR16161
Depends on D3898
Reviewer note: the last commit in this PR is TRAVIS related and was therefore modified to our code base.