0.20.6

Added

• Much better code generation for segmented scans with vectorisable
  operators.

Fixed

• Fixes to extremely exotic GPU scans involving array operators.

• Missing alias tracking led to invalid rewrites, causing a compiler
  crash (#1499).

• Top-level bindings with existential sizes were mishandled (#1500, #1501).

• A variety of memory leaks in the multicore backend, mostly (or
  perhaps exclusively) centered around context freeing or failing
  programs - this should not have affected many people.

• Various fixes to f16 handling in the GPU backends.