Fix other environment cases

[vinistock]

I found a few issues in the implementation related to aref_field, method_add_block and nested arguments.

ARefField

This boils down to a var_ref, so we were actually counting them twice. Also, visit_aref_field would be unnecessarily complicated if the collection was a Call, which would need special treatment. Simply removing that visit makes it so that we only go through var_refs once and works when the collection is a Call

MethodAddBlock

These nodes have two parts: the call and the block. Only the block requires a fresh environment block, the call occurs in the same environment that the node is found. E.g.:

object = MyObject.new
# object is inside the call part of method_add_block, but it happens in the current environment
object["attributes"].find do |a|
  # only inside this block do we need a fresh environment
  a["field"] == "expected"
end

Nested arguments

We weren't handling the case where required positional arguments were "nested". E.g.:

[].each do |one, (two, (three, four))|
end

I used recursion to fix this one because (to my surprise) it seems that Ruby allows you to have as many layers as desired. I assume most cases will only have one level, but better to model what Ruby allows you to do.