Macros: automatically resolve alias in types

[asterite]

Fixes #4301

This makes is so that if T is a (non-recursive) alias type, using T in macros automatically resolved to the aliased type. With this we can now do (for example):

puts "All primitive integers"
{% for type in Int::Primitive.union_types %}
  puts {{type}}
{% end %}

This can greatly simply code. For example here:

crystal/src/json/from_json.cr

Line 69 in 74050a4

{% for type in %w(Int8 Int16 Int32 Int64 UInt8 UInt16 UInt32 UInt64) %}

An alternative implementation would be to add a macro method to solve an alias type. But alias types in regular code can never be reached by the user: trying to use one immediately uses the underlying type. So doing the same thing in macros is consistent. For example:

alias Foo = Int32

There's simply now way to make a program print "Foo" at runtime by using that alias, because trying to use it will solve to Int32.