Move even more low level code into PDF::COre

Squashed commit of the following:

commit 168c44f034fa38ed4845854e6aeae400b1b64dea
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Sep 15 15:25:26 2014 -0400

    Continue to shrink renderer's API surface

commit 0c8befe470ed79bdebc4395fbdb083120474eabc
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Sat Sep 13 20:53:31 2014 -0400

    Make Prawn::Document::Internals mixin smaller

commit 0071a689c39741308db092a3ddd399856e56a4ff
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Sat Sep 13 20:33:18 2014 -0400

    Use explicit names for delegated methods

commit a77e9aa5b81b404bbd8d3be6d36316dd2e13c6f3
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Sat Sep 13 20:28:30 2014 -0400

    Remove some monkey patches and push more down to PDF::Core

commit a42d70d867f8efd5fd5407dac0c3745c64cf5a1b
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Sat Sep 13 19:03:00 2014 -0400

    Rough integration of PDF::Core::Renderer