WIP: make SandboxedPath internals always Abs

[srghma]

Description of the change

this pr is on top of #52

what is here:

1. only appeand, no go up SafeAppend <///> for SandboxedPath
2. make SandboxedPath not have Rel in type - always Abs to fix all sandbox methods should return Abs Paths? #53
3. add Pathy.Name.basename

Where I use it? here https://github.com/srghma/purescript-pathy-node , can move to purescript-contrib

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[garyb]

Sandboxing is only meant to guarantee that a user provided path is not escaping some absolute context, it never claimed to make the path absolute as far as I know. But I can see where you're coming from, it does make sense that unsandboxing/printing a sandboxed path would produce an absolute path, unfortunately I don't think that's entirely harmless just now - we'd need to figure out and have the windows absolute paths working correctly too.

Also, since I don't think I've said it elsewhere, thanks for all the work you're putting into things all over the place!