Linearize singleton ancestors

[vinistock]

Motivation

Closes #1939, closes #1938, closes #1333

This PR adds lazy singleton ancestor linearization, which allows us to properly handle constants, methods and instance variables that are inherited by a singleton class.

Implementation

1. We already had lazy creation of singleton classes spreading across three different places. Moved that to the Index, which is the entity that's actually concerned with those. Started using it in RBSIndexer and the regular declaration listener
2. Removed the concept of extend. An extend is nothing but a convenience to include something in the singleton class. If we simply treat all extends as singleton class inclusions, the entire algorithm works as is and we don't have to handle another type of operation
3. Finally, the most important part is how to lazily determine the ancestors based on how many levels of singleton we are checking. Here's how the algorithm works:

For each level of singleton, the ancestors are: the linearization of the same level of singleton + the linearization of the level - 1 of the Class class. For example:

class Foo; end
class Bar < Foo; end

# First singleton
Bar.singleton_class.ancestors
# => [Bar::<Class:Bar>,
#     Foo::<Class:Foo>,
#     Object::<Class:Object>,
#     BasicObject::<Class:BasicObject>, # <<<< Until here, it's the ancestors of <Class:Bar>
#     Class, Module, Object, Kernel, BasicObject] # <<< Ancestors of Class

# Second singleton
Bar.singleton_class.singleton_class.ancestors
# => [Bar::<Class:Bar>::<Class:<Class:Bar>>,
#     Foo::<Class:Foo>::<Class:<Class:Foo>>,
#     Object::<Class:Object>::<Class:<Class:Object>>,
#     BasicObject::<Class:BasicObject>::<Class:<Class:BasicObject>>,
#     ^^^^^^^ until here, it's the ancestors of <Class:<Class:Bar>>
#     Class::<Class:Class>, Module::<Class:Module>, Object::<Class:Object>,
#     BasicObject::<Class:BasicObject>,
#     ^^^^^^^ then the ancestors of <Class:Class> (the singleton level - 1)
#     Class, Module, Object, Kernel, BasicObject]
#     ^^^^^^^ And finally the ancestors of Class

Automated Tests

Added tests.

Manual Tests

1. Launch LSP on this branch
2. Type File. in any file
3. Verify that you see methods inherited from the IO class, such as the method pipe