Optimize allocation for node dependencies and observers in compiler

[straight-shoota]

Most of the time, dependencies and observers arrays only have one element. Allocating an array for a single element is inefficient and can be optimized.

This was originally implemented in #12405 changing the type from Array(ASTNode) | Nil to a union Array(ASTNode) | ASTNode | Nil where single elements are represented as ASTNode directly.
But that broke the interpreter (#12769), so it was reverted in #12849.

This optimization probably still makes sense, but we need to figure out what's breaking and fix that.