Added intersperse and cycle to utils.ml
Fix: same namespace for linear and non-linear vars
Exposing auxiliary functions for pretty printing lambdas
Exposing the id_to_sym function of signatures
Remove backwards application from Diagram

It is already contained in the OCaml pervasives.
Added reframe to Diagram
Added get_font_extents to Diagram
Added functions to Diagram interface
Added bg_color to Diagram
Added show_extents to Diagram for debugging purposes
Added Tree module for n-ary trees
realize_show fully working