Some RGBLINK refactoring

- Consistently refer to `Section` fragments/unions as "pieces" (renaming `.nextu`)
- Remove `Symbol`'s `.label()` accessors (use `std::get<Label>`)
- Move some `Label`-related logic into `Symbol` methods

include/link/section.hpp

@@ -48,7 +48,7 @@ struct Section {
 	// Extra info computed during linking
 	std::vector<Symbol> *fileSymbols;
 	std::vector<Symbol *> symbols;
-	std::unique_ptr<Section> nextu; // The next "component" of this unionized sect
+	std::unique_ptr<Section> nextPiece; // The next fragment or union "piece" of this section
 };
 
 // Execute a callback for each section currently registered.

include/link/symbol.hpp

@@ -33,8 +33,8 @@ struct Symbol {
 	    >
 	    data;
 
-	Label &label() { return std::get<Label>(data); }
-	Label const &label() const { return std::get<Label>(data); }
+	void linkToSection(Section &section);
+	void fixSectionOffset();
 };
 
 void sym_ForEach(void (*callback)(Symbol &));