Implement Method Return Types

[faultyserver]

This is work that I did a number of months ago and have completely forgotten if it was finished or not. However, looking at the diff and seeing the tests all pass, it seems mostly complete to me, so I'm creating this PR.

In general, this PR does 4 things:

• Adds support for return type specifications on all forms of methods.
• Adds a true base type Object for representing instances rather than just static Types. Type is now a subclass of Object, and any type definitions will inherit from it, while instances will not.
• Adds return type specifications to most methods in the standard library.
• Adds support for Type Unions following the Crystal syntax style of TypeA | TypeB. Anywhere a type restriction is given, a union should be able to take its place.

Importantly, this PR does not implement return type checking. That is, specifying a return type on a method does not actually cause anything to happen in the interpreter. It is purely decorative for now and relies on developers honoring their function signatures manually. Eventually, this will get used by https://github.com/faultysever/myst-typecheck for pre-execution type checking, and will be used by the interpreter to enforce that those restrictions are satisfied at runtime.

[bmulvihill]

@faultyserver Unions don't appear to be working correctly, unless my syntax is wrong here:

deftype Test
  def initialize
  end

  def union(x : String | Integer)
    STDOUT.puts(x)
  end
end

test = %Test{}

test.union("hi")
test.union(1)

produces: Uncaught Exception: No clause matches with given arguments: ["hi"]

[faultyserver]

@bmulvihill Your code is correct and should work. The Matcher needs to be updated to handle Unions as patterns.

[faultyserver]

@bmulvihill The latest commits should add the necessary support in the interpreter for your example to work.