Parsing should not depend on `std`

There are some instances where parsing of structs (`core::str::FromStr`) makes use of the implementations provided by the standard library. Problematically, these are defined in `std` and not in `core` such that this depends on the `std` feature. This makes the trait implementations inconsistent and makes the dependency quite a lot heavier.

Instead, such parsing should be implemented in a standalone version (possibly with a dedicated, small dependency).

Look for structs implementing `core::str::FromStr` that rely on `std` parsing. Identify which types are affected and examine their current parsing logic. Research small, no-std parsing crates or consider implementing a minimal parser. Test changes by ensuring the crate still builds without the `std` feature.

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