Validate duplicate type aliases

[pocke]

An environment should reject duplicate type aliases, but actually, it just ignores the duplication due to a forgotten raise.

This PR fixes the validation to raise an error on duplicate type aliases, like the following.

type foo = String
type foo = Integer

Additional Information

Interface has the same problem.

rbs/lib/rbs/environment.rb

Lines 364 to 371 in 0dd11d3

	when AST::Declarations::Interface
	name = decl.name.with_prefix(namespace)
	if interface_entry = interface_decls[name]
	DuplicatedDeclarationError.new(name, decl, interface_entry.decl)
	end
	interface_decls[name] = InterfaceEntry.new(name: name, decl: decl, outer: outer)

For example, the second _I conceals the first one in the following example.

interface _I
  def foo: () -> void
end

interface _I
  def bar: () -> void
end

module M
  include _I
end

We can confirm this problem with the following command.

# It should print `foo` and `bar`,
# but actually it prints only `bar`.
$ rbs -I. methods --no-inherit M
bar (public)

I think the interface should behave the same as the open class, instead of rejecting duplicated interfaces.
I will create a PR to fix the interface problem soon.

[pocke]

I and @soutaro discussed duplicated interfaces. It should cause an error also.
So I made it causes a duplication error on interfaces too. 388e6c8

[soutaro]

Thanks! 💦