0.5.3: Metal backend (macOS with GPUs including Apple Silicon)

WARNING: this release's test suite depends on an unreleased fix to PrintBox at the time of the release. I'm not planning on submitting it to the opam repository.

Highlights:

• What it says on the tin: GPUs like the M1, ..., M4.
• Got rid of Stdlib.Format as I found it error prone and hard to debug.
• A simple test for logging from within kernels in the test suite.

From the changelog:

Added

• The Metal framework backend (Apple Silicon).
• Setting debug_log_to_stream_files to neatly keep logs from routine execution in their separate files.
• Settings clean_up_artifacts_on_startup, prefer_backend_uniformity.
• Tools directory and the minised tool: regexp replacement file rewrite.
• Directory arrayjit/bin and executable read_config for extracting OCANNL configuration into txt files.

Changed

• Removed initialize and is_initialized from the backend API; instead, backends should be initialized on functor application. The functors now take config as argument.
• More descriptive identifier names in C-syntax code in case of name conflicts.
• Changed the backend config name cc to multicore_cc for consistency.
• Migrated out of Stdlib.Format to PPrint for all structured formatting.
• Migrated stdout capture to thread-based (domain-based actually); for Windows compatibility but also much more robust for large logs.

Fixed

• Avoid conflicts with C math function names like fma.
• Satur01_gate had wrong semantics.