Add documentation and small improvements to core:container/topological_sort

[thetarnav]

I've used the topological sort package recently and noticed that it doesn't follow the same "quality" as other and newer packages.
This PR:

• adds small docs for each procedure
• adds doc.odin file with a small usage example
• Reuses Sorter.relations.allocator for allocating Relations.dependents map instead of storing additional allocator in Sorter
• Adds allocator := context.allocator param to init and sort
• Adds err: runtime.Allocator_Error result to sort
• Returns slices from sort instead of dynamic arrays
• Allocates a single continuous buffer for sorted and cycled slices instead of having two separate dynamic arrays
• Uses @require instead of _ :: pkg
• Adds require_results attr to sort

The change should be mostly backwards compatible with the exception of added err: runtime.Allocator_Error result which needs to be handled.