Use covariant return types in non-generated clone methods

src/main/java/org/sablecc/sablecc/CharSet.java

src/main/java/org/sablecc/sablecc/IntSet.java

src/main/java/org/sablecc/sablecc/LR0Item.java

src/main/java/org/sablecc/sablecc/LR0ItemSet.java

src/main/java/org/sablecc/sablecc/LR1Item.java

src/main/java/org/sablecc/sablecc/LR1ItemSet.java

src/main/java/org/sablecc/sablecc/NFA.java

src/main/java/org/sablecc/sablecc/SymbolSet.java