[FEATURE REQUEST] Refactoring of NativeCalls

[lucaneg]

Description
While the idea of supporting language constructs through NativeCalls seemed appropriate, the only difference from "regular" expressions is that it was easier to support n-ary expressions. We should remove NativeCalls entirely and transform them into Expression instances.

After this, we should add a generic NaryExpression that serves as a base class for what we are now supporting through native calls.

As a final note, to make expression evaluation more generic, we should also add the option of changing the evaluation order of the parameters of an expression: imposing left-to-right recursive evaluation, as well right-before-left evaluation in assignments, is too restrictive.