Use explicit getters to access all fields

[Fingercomp]

For example, replace uses of LeafNode.parentNode outside that class with LeafNode:getParent().

Though the basic idea is simple, it requires changing a ton of the code, and is very error-prone. But this kind of thing is better to be done earlier than later.

Expected advantages:

• Increased perfomance when switching from Foo.__getters:bar to Foo:getBar(). __getters are implemented using metamethods, so, when an object is flattened by calling object:optimize(), they aren't added to the object table. Which means that every access to the getters requires traversing the inheritance tree.
• Errors detected earlier. node.parent (expected node.parentNode) returns nil all the time. This hides the real error, as node.parentNode can also return nil if the node has no parent (is a root node). I've already spent a few days debugging such problems.
• Makes it explicit that all fields of a class are considered implementation details, and aren't supposed to be accessed directly by the external users, but via the means of getFoo and setFoo methods.
  • Also makes it easier to preserve the invariants.
• Behavior of getters that have side-effects is made less suprising.

Expected disadvantages:

• Decreased perfomance when switching from Foo.field (just a table index) to Foo:getField() (usually 2 table indexes, and a call). I don't think the added overhead here are too high to be ignored, but if it does become a bottleneck, the field could just be accessed directly.