Clone macro default argument before macro expansion

[makenowjust]

Currently macro default argument is not cloned on expanding, so it causes unexpected behavior when mutating default argument. For example:

macro foo(foo = [] of String)
  {% foo.push "foo" %}
  p {{foo}}
end

foo
foo

I expected the below because method's default value is evaluated on every call:

["foo"]
["foo"]

However, crystal run outputs this:

["foo"]
["foo", "foo"]

This PR fixes this behavior.

[Sija]

Will it keep values which shouldn't be cloned intact, like in the example below?

class Foo
  FOO = [] of String

  # here, `baz` argument shouldn't be `#clone`d.
  macro bar(baz = FOO)
    {% baz.resolve.push "foo" %}
  end
end

Foo.bar
Foo.bar

# CORRECT
pp Foo::FOO # => ["foo", "foo"]

# INCORRECT
pp Foo::FOO # => ["foo"]

[makenowjust]

ping.

[RX14]

Given @Sija's code sample works after this PR.

[asterite]

Yes, I tried that code and it works fine.