Implement tuple and named tuple covariance

[asterite]

Fixes #357

This also creates tuples/named-tuples of unions instead of unions of tuples/named-tuples, if the size/keys match. So this is possible:

#1.
ary = [] of {Int32 | String, Char}
ary << {1, 'a'}
ary << {"hello", 'a'}

#2.
class Foo; end
class Bar < Foo; end

ary = [] of {Foo, Foo}
ary << {Foo.new, Bar.new}

And:

#1.
ary = [] of {foo: Int32 | String, bar: Char}
ary << {foo: 1, bar: 'a'}
ary << {bar: 'a', foo: "hello"}

#2.
class Foo; end
class Bar < Foo; end

ary = [] of {foo: Foo, bar: Foo}
ary << {foo: Foo.new, bar: Bar.new}

The snippets above don't work right now, but they do with this change.