arrowbricks v5.1.0

• Linux: buffers of 1 MiB or more now come straight from mmap and go back
  to the OS when freed (src/alloc.rs, a scoped global allocator for this
  extension only; the host process's malloc is untouched, other platforms
  are unchanged). glibc raises its mmap threshold as large blocks are freed,
  so repeated queries in one long-lived process kept the memory: idle RSS
  grew from 200 MB to 1.4 GB over five queries of the same 500k-row table.
  Measured on a real warehouse (5 workloads, Thrift and SEA, LZ4-compressed
  results, interleaved against 5.0.3): peak RSS -41% to -64% on the larger
  results, idle RSS after repeated queries about -80%, time unchanged within
  noise. Tiny results are unaffected. No API changes.
• Correction to 5.0.2: its memory and speed figures were measured against a
  local mock warehouse serving uncompressed bodies. Against a real warehouse,
  where results arrive LZ4-compressed, 5.0.2 and 5.0.3 behave like 4.0.0 on
  fetch time and memory; the gain described here comes from 5.1.0.