[DRAFT FOR CARPET] Reuse immutable instances, make some functions reuse existing code, and some minor changes

[altrisi]

All minor changes:

• Makes NumericValue, StringValue and subclasses return themselves on clone. At the end of the day, all they did was make an instance with the same thing inside. I just thought why have those duped instances. This also changes NullValue.equals() to be reference equality
• Makes some code (around ValueConversions) reuse existing methods
  • of(Identifier) now uses simplify()
  • Moves some duped code to call ValueConversions
  • Makes of(BlockPos) and of(Vec3d) call ListValue.fromTriple
  • Makes ListValue.fromTriple(int) accept longs instead and use long-specific NumericValue constructor
• Makes slot id to lv map be int specific
• Makes constants map static so each expression doesn't hold a copy
• Makes Value.NULL, Value.TRUE, etc constants final

[altrisi]

Pretty much doesn't work, because scarpet relies in specific values for assigning variables...