Equelle kitchen sink

This pull request adds the Equelle "kitchen sink", a web application that includes:
 - A code editor with syntax highlighting and code completion for the Equelle language, based on CodeMirror
 - Interactive specification of the grid and inputs to an Equelle program
 - 3D visualization capabilities of Equelle program outputs, based on XTK

Compilation of Equelle code to an executable, and running of this executable is done by a server that can be set up using a bootstrapping bash script.

Changes in this pull request are mainly to files in a new subdirectory "webdemo". Some changes of the Equelle compiler is done to analyze built-in functions of the Equelle language and inputs and outputs of an Equelle program (added the "--dump io/symboltable" argument to the compiler).

@atgeirr, @babrodtk and @alessandrorossini, will you take a look at this?