2023 Release

Highlights include:
* Size-change graphs are now matrices, faithfully implementing [Lee, Jones, And
    Ben-Amram; 2001]
* Elaborator scripts can now access project files, allowing for type-providers
    and similar
* Warnings on conflicting fixity declarations along with `%hide` support for
    these
* Numerous doc and error message enhancements, bug fixes, performance
    improvements, and much more (see the CHANGELOG for full details).

Update version number in CHANGELOG (#2734)

Disable -Xcheck-hashes, at least for the moment (#1940)

* Version increment to 0.5.1

This is to remove the requirement on Chez >9.5

* Disable -Xcheck-hases, at least for the moment

If we're going to have this as an option, we need to have a portable way
of finding a sha256sum command. At the moment, we might find a command,
but different versions accept different options. We should at least
allow setting it via an environment variable, and we certainly shouldn't
fail if running the command fails.

* Update bootstrap code ready for 0.5.1 release

Version 0.5.0 (#1931)

* Update version numbers and bootstrap scheme

* Use wall clock time for search timeouts

That was always the intention in any case, rather than the process time.

Merge pull request #1599 from edwinb/prepare-040

Changes for next release, 0.4.0

Merge pull request #921 from edwinb/version030

Update version numbers and bootstrap code

Compatibility fix (again)

Need to use named implementations to avoid a clash between current
version and this version, so that it builds with *both* the bootstrap
and the previous version.

Release version 0.2.1

Release version 0.2.0