Fix top level constant resolution

[vinistock]

We got the constant resolution algorithm slightly wrong in terms of the top level fallback. This PR fixes it.

1. First, there's no top level fallback when inside a class. All constants end up being available through inheritance. It looks like there's a top level fallback because not inheriting from Object is pretty uncommon, but you can see that this is the case because this crashes:

# This crashes because `Foo` doesn't have `Object` in its ancestor chain,
# which is the owner of the `String` constant
class Foo < BasicObject
  String
end

And when using constants at the top level of a script, it works because <main> is an instance of Object.

# This doesn't crash, but not because there's a top level fallback. It's simply
# because <main> is `Object`
String

2. The fallback does exist for modules. However, it's not a simple direct member access. Ruby will actually search the ancestors of Object. For example, this works:

module Kernel
  FOO = 1
end

module Bar
  puts FOO
end

If Ruby can't find the constant in lexical scopes or ancestors of the surrounding nesting, it searches the ancestors of Object. The same exact behaviour occurs for top level references like ::FOO - even inside classes!

module Kernel
  FOO = 1
end

class Bar < BasicObject
  ::FOO # The top level reference will make Ruby search in the ancestors of Object
end

[vinistock]

Warning

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• Fix top level constant resolution #729 : 2 dependent PRs (#730 , #731 ) 👈 (View in Graphite)
• Fix default parent classes #728
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[st0012]

Can you also update ruby-behaviours doc with this info?

[vinistock]

@st0012 done #731.