Renaming the module from RubyVM::DebugInspector to DebugInspector (required for TruffleRuby support)

[eregon]

TruffleRuby implements the debug_inspector C API since a few releases.
So it should be easy to get this gem working on TruffleRuby.

However, there is one big issue, this gem defines RubyVM::DebugInspector and not DebugInspector.
RubyVM should really never be defined on any Ruby but CRuby, otherwise various other gems fail as they assume that if RubyVM is defined then CRuby-specific APIs under RubyVM are defined too:
https://github.com/search?q=defined%3F%28RubyVM%29&type=code
https://cs.github.com/?q=defined%3F%28RubyVM%29
For example, there is lots of code like this:

    if defined?(RubyVM)
      RubyVM::InstructionSequence...

From #35

So, could this gem define top-level DebugInspector?
It could still define RubyVM::DebugInspector on CRuby (or if defined?(RubyVM)) for compatibility.
I think it's be best to use deprecate_constant though so usages of RubyVM::DebugInspector get replaced by DebugInspector.

cc @bjfish