Add `SubsectionData` enum

`Subsection` and `Sym` are structurally similar. Each has a `kind` and `data` field. `Sym` _also_ has a `parse()` method, which returns a `SymData` enum that covers the logical cases enumerated by the constants in `SymKind`. It'd be nice to have a corresponding `Subsection::parse()` method, which would return a `SubsectionData` enum for easy matching. 