Translate T.cast and T.must into RBS comments

[Morriar]

This will now allow to translate T.must into RBS comments so this:

x = T.cast(ARGV.first, String)

becomes this:

x = ARGV.first #: as String

And T.must so this:

x = T.must(ARGV.first)

becomes this:

x = ARGV.first #: as !nil

[paracycle]

What happens to constructs like:

foo = T.cast(T.must(T.must(bar).baz).quux, String)

If we are not handling them then it might be good to make that explicit in the option messages and with tests.

[paracycle]

The inclusion of nil here looks out of place to me:

Suggested change

	option :let, type: :boolean, desc: "Translate `T.let` to `nil #: String?`", default: true
	option :let, type: :boolean, desc: "Translate `T.let` to `#: String`", default: true

I think a better message would be:

Suggested change

	option :let, type: :boolean, desc: "Translate `T.let` to `nil #: String?`", default: true
	option :let, type: :boolean, desc: "Translate `T.let(x, String)` to `x #: String?`", default: true

[paracycle]

If my suggestion above is accepted, then better messages would be:

Suggested change

	option :cast, type: :boolean, desc: "Translate `T.cast` to `# as String`", default: true
	option :must, type: :boolean, desc: "Translate `T.must` to `# as not nil`", default: true
	option :cast, type: :boolean, desc: "Translate `T.cast(x, String)` to `x # as String`", default: true
	option :must, type: :boolean, desc: "Translate `T.must(x)` to `x # as !nil`", default: true

[paracycle]

Suggested change

	option :cast, type: :boolean, desc: "Translate `T.cast` to `# as String`", default: true
	option :must, type: :boolean, desc: "Translate `T.must` to `# as not nil`", default: true
	option :cast, type: :boolean, desc: "Translate `T.cast` to `# as String`", default: true
	option :must, type: :boolean, desc: "Translate `T.must` to `# as !nil`", default: true

[paracycle]

It might be a good idea to do:

Suggested change

	@annotation_methods.freeze

here.