WIP on a bunch of things
Add slab_allocator and update other two allocators

Don't store memory inside the allocator unless we have to.
WIP
Remove countdown SQUASHME
Work up objstack
Use std::align to make objstack_allocator trivial
Poke objstack_allocator more. Still broke
Add simple FILE* wrapper
Kill internal_allocator. Impossible to do portably
Credit Modern C++ Design for fixed_allocator
Remove owning-semantics from obj_allocator
Flesh out the forwarding for stdio_file
House-keeping

Add license notices to files

Fix include paths to be relative to libcpp-util root

Fix include guards
Add allocator for std::malloc
Make fixed_allocator::chunk noexcept move
Rewrite strprintf module to use variadic templates
Fix typo in mem/util.h
Add stub for chunk allocation in fixed_allocator