Consistent and clean `Context` implementation

[raviqqe]

• Should we force users to pass around Melior's own Context type and disallow the use of ContextRef<'c>?
• Should we remove &Context and turn everything into ContextRef<'c> as discussed in the following issues in Inkwell?
  • Types derived from ContextRef are always shortlived TheDan64/inkwell#165

[raviqqe]

The last thing we should consider and potentially improve is the argument to callback_initialize in the external_pass module.