feat: allow no_std

[stifskere]

Pre-submission Checklist

• I've checked existing issues and pull requests
• I've read the Code of Conduct
• I've implemented tests for my changes
• I've listed all my changes in the Changes section

Changes

• Gate STD under the std feature.
• Gate dyn_path under any of std feature or alloc feature.
• Enable the std feature by default.
• Re-export alloc in the case of no having the std feature and alloc enabled.
• Change all the ::std paths into ::core or $crate::alloc.