RubyVM usage

Hello, it seems that currently rbs uses RubyVM::AbstractSyntaxTree in a couple places:
https://github.com/ruby/rbs/search?q=RubyVM&unscoped_q=RubyVM

However, RubyVM by design is MRI-only and is expected to not exist as a constant on alternative Ruby implementations (documentation).

Do you plan to address this somehow?
If AbstractSyntaxTree is needed by rbs then I think it's time to move it somewhere outside of RubyVM.

For instance, when running on TruffleRuby:

% rbs prototype rb lib/person.rb lib/email.rb lib/phone.rb
/Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/prototype/rb.rb:55:in `const_missing': uninitialized constant RBS::Prototype::RB::RubyVM (NameError)
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/prototype/rb.rb:55:in `parse'
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/cli.rb:629:in `block in run_prototype_file'
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/cli.rb:628:in `each'
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/cli.rb:628:in `run_prototype_file'
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/cli.rb:526:in `run_prototype'
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/lib/rbs/cli.rb:92:in `run'
	from /Users/bfish/Documents/truffleruby-ws/truffleruby/mxbuild/truffleruby-native/jre/languages/ruby/lib/gems/gems/rbs-0.7.0/exe/rbs:7:in `<top (required)>'
	from <internal:core> core/kernel.rb:395:in `load'
	from <internal:core> core/kernel.rb:395:in `load'
	from /Users/bfish/.rbenv/versions/tr-native/bin/rbs:23:in `<main>'

cc @bjfish

Relates to https://github.com/oracle/truffleruby/issues/1671

Start by reproducing the failure with rbs prototype rb lib/person.rb lib/email.rb lib/phone.rb on TruffleRuby, then inspect the RubyVM references, especially lib/rbs/prototype/rb.rb around line 55 and the prototype entry point in lib/rbs/cli.rb. Done means the prototype behavior and parsing approach are defined for Ruby implementations where RubyVM is unavailable.