Further improvements to memory.c. (#1625)

Further improvements to memory.c. (#1625)

- Compiler TLS is now used only used when the compiler supports it
- If compiler TLS is unsupported, we use platform-specific TLS
- Only one variable (an index) is now in TLS
- We only access TLS once per alloc, and never when freeing
- Allocation / release info is now stored within the allocation itself, by
  over-allocating; this saves having external structures do the bookkeeping, and
  reduces some of the redundant data that was being stored (such as addresses)
- We never hit the alloc lock when not using SMP or when using OpenMP (that was
  my fault)
- Now that there are fewer tracking structures I think this is a bit easier to
  read than before