Don't emit final |nil| for include initialization methods

It's useless.
Y2R::AST::YCP::ModuleBlock: s/initialize/main

We changed the method name, we should also change corresponding
identifiers in the code.
Y2R::Parser: Pass options already to |initialize|, not to |parse|

This change allows us to store the options in an instance variable,
which makes their storage inside |Y2R::Parser::Context| redundant. This
makes the |Y2R::Parser::Context| class mostly useless and we can get rid
of it (coming soon).
Get rid of |Y2R::Parser::Context|

It's mostly useless now.
Rename |Y2R::AST::Ruby::Context| to |Y2R::AST::Ruby::EmitterContext|

It's more descriptive.
Rename |Y2R::AST::YCP::Context| to |Y2R::AST::YCP::CompilerContext|

It's more descriptive.
Combine options passed to |CompilerContext| to one |options| property

This will make any changes in compiler options much easier as less
places will need to be touched.
Make |CompilerContext| an instance of |OpenStruct|

This simplifies the class a bit.
Refactor CompilerContext#inside

The form with a |dup| call is more similar to methods in
|EmitterContext| and avoids repeating all context attributes.