Bypass slab allocator needs to happen in smartMallocSize

Summary: In dbg builds, routing to smartAllocSizeBig inside slabAlloc causes us
to wrap the object in multiple layers of debug headers, which aren't untangled
correctly by smartFreeSize.

Reviewed By: @mclarkelauer, @edwinsmith

Differential Revision: D1695774

Signature: t1:1695774:1416615918:6528f4a17bc14850eb2920dec36edd13bfa4a551
Fix build when using system installed lz4 library instead of third-party

Summary: Without this patch, the build fails with the following error:

```
+ /usr/bin/ocamlopt.opt heap/hh_shared.o utils/realpath.o inotify/inotify_stubs.o utils/nproc.o hhi/hhi_elf.o utils/get_build_id.gen.o utils/get_build_id.o -cclib -lelf -cclib -lpthread -cclib -L/usr/lib/x86_64-linux-gnu -cclib -L/usr/lib/x86_64-linux-gnu unix.cmxa str.cmxa avl/monoidAvl.cmx utils/hh_json.cmx utils/ident.cmx utils/sys_utils.cmx utils/utils.cmx utils/relative_path.cmx utils/pos.cmx deps/fileInfo.cmx deps/typing_graph.cmx deps/typing_deps.cmx inotify/inotify.cmx fsnotify_linux/fsnotify.cmx dfind/dfindEnv.cmx dfind/dfindMaybe.cmx dfind/dfindAddFile.cmx utils/sysConfig.cmx utils/tmp.cmx dfind/dfindServer.cmx dfind/dfindLib.cmx globals/autocomplete.cmx globals/find_refs.cmx globals/ide.cmx globals/serverConfig.cmx heap/globalStorage.cmx heap/prefix.cmx heap/value.cmx heap/sharedMem.cmx utils/path.cmx utils/shell.cmx hhi/hhi.cmx parsing/namespace_env.cmx parsing/ast.cmx utils/errors.cmx naming/naming_ast_helpers.cmx naming/naming
Closes https://github.com/facebook/hhvm/pull/4302

Reviewed By: @jwatzman

Differential Revision: D1697207

Signature: t1:1697207:1416852626:cd5878ac8fc915bc6571b7250b5f40faeea0ea96
Fix potential memory corruption during ArrayData::set

Summary: SetM can trigger a destructor, which can reenter the VM and take
actions that cause the array base to change (e.g., an upgrade from packed to
mixed).  So, after the cellSet it's not valid to keep touching the array's
memory, since it may have been freed.

Reviewed By: @markw65

Differential Revision: D1697915