RSpecRenderer rewrite: Extract the Code module

The Code module contains utility functions for manipulating code. It is
cleaner to have these separated. Also, the renderer will be split into
multiple classes soon. All these classes will use some of the utility
functions, so they can't stay in the renderer.
RSpecRenderer rewrite: Extract the It class

This decouples building the "it" block from its rendering.
RSpecRenderer rewrite: Extract the Describe class

This decouples building the "describe" block from its rendering.
RSpecRenderer rewrite: Remove now useless parameter of Code.indent