refactor: deprecate str package and move to stdlib

[kruskall]

Problem Description

The standard library now includes dedicated maps and slices packages, which provide the functionality currently covered by the custom str, strset, and ToSlice utilities.

Impact

Maintaining these custom packages duplicates functionality that is already available in the Go standard library. This adds unnecessary code bloat, creates extra maintenance work, and can lead to inconsistencies between the custom implementations and the officially supported ones.

Suggested Solution

Deprecate the str, strset, and ToSlice package. Update the codebase to use the corresponding functions from the standard library’s maps and slices packages instead, and remove the now‑redundant custom implementations.