Rework loading API

[bluebear94]

This adds two methods to LoadSave to replace the old Load and loadOrBlank methods:

// Take `Class<OBJ>` for class instead of raw `Class`
// Remove unused `newobj` parameter

public static <OBJ> OBJ load(Class<OBJ> clazz, CompoundTag nbt, String loc);
// Take `Supplier<OBJ>` for `blank` to avoid creating a new object unless needed
public static <OBJ> OBJ loadOrBlank(Class<OBJ> clazz, CompoundTag nbt, String loc, Supplier<OBJ> blank);

Unlike in #3, the old methods are only deprecated to avoid causing immediate API breakage.