chore: slices cleanup

[superlinkx]

Description

Refocuses our internal slices module as an extension of the newly available Go 1.21+ stdlib slices package. This involved renaming our slices module to slicesext and removing functions that stdlib now provides. All features that our slicesext module provide that the stdlib doesn't were kept as is.

This also backports an upcoming stdlib slices function into our extension module: Append!

Motivation and Context

• Our slices module became a bit unwieldy after the upgrade to Go 1.21, since it directly collides with the name of the stdlib package with a different set of features, but some overlap
• Removing functions that we maintain in favor of stdlib variants seemed like good hygiene
• We've been needing a proper slice append function for a good long time, and with it right around the corner, it seemed prudent to start moving towards its use. slicesext uses the current version of this function since it's a fairly stable piece of code and isn't expected to have any API changes between now and Go 1.22 full release. When we eventually upgrade to Go 1.22, we can delete our copy as well as the copied test from our extension, but for now it lets us start using Append with a clean refactor path.

How Has This Been Tested?

All tests continue to pass. New tests added for new function (stolen and slightly tweaked from Go source).

Types of changes

• Chore (a change that does not modify the application functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Documentation updates are needed, and have been made accordingly.
• I have added and/or updated tests to cover my changes.
• All new and existing tests passed.
• My changes include a database migration.

[zinic]

No comments. Merge when passing.

[sircodemane]

Lgtm