0.15.4

Added

• futhark pkg now shells out to curl for HTTP requests.

• futhark doc now supports proper GitHub-flavored Markdown, as it
  uses the cmark-gfm library internally.

• Top-level constants are now executed only once per program
  instance. This matters when Futhark is used to generate library
  code.

• futhark autotune is better at handling degrees of parallelism
  that assume multiple magnitudes during a single run.

• futhark pkg now uses curl to retrieve packages.

• Type errors are now printed in red for better legibility (thanks
  to @mxxo!).

Fixed

• Fixed incorrect handling of opaques in entry point return types.

• futhark pkg now works properly with GitLab (#899).