SpeedscopeRenderer: remove dead code, update docs
SpeedscopeRenderer: remove dead code, update docs

This commit revises the SpeedscopeRenderer class by removing a lot of
dead code and updating docstrings and comments in the class and its
auxiliary classes.

joerick/pyinstrument,joerick/pyinstrument,joerick/pyinstrument,joerick/pyinstrument,joerick/pyinstrument,joerick/pyinstrument