Move self handling to variable lookup

[jg-rp]

This PR shows an alternative implementation of self by removing SelfDrop and moving handling of self from Context to VariableLookup.

This approach changes self semantics to fix the issue identified in #2080. Instead of conditionally replacing self with a drop, we now fall back to top-level variable resolution if self[key] does not exist.

A comforting side effect is that self can no longer be aliased with {% assign this = self %}, or be passed as an argument {% render 'snippet', this: self %} (this does not cause too much of a problem because SelfDrop.context is always replaced with the current context).

(Edit: My assumptions about the intended scoping rules for self might not be quite right.)