Replace custom hex parsing with hex-conservative crate

Add hex-conservative as a dependency and refactor hex parsing code to use it
instead of the custom implementation. Update FromHex trait to use associated
error types and standardize hex decoding across the codebase.

Author: Velnbur

Cargo.toml

@@ -32,6 +32,7 @@ serde_json = { version = "1.0", optional = true }
 actual-serde = { package = "serde", version = "1.0.103", features = [
     "derive",
 ], optional = true }
+hex-conservative = "1.0.0"
 
 
 [target.wasm32-unknown-unknown.dev-dependencies]