Proof-of-Concept WebAssembly port of libfive

Demos:

• example-cxx-libfive-basic
• example-cxx-libfive-to-stl — renders a sphere (of requested radius) with three.js from an STL file exported from a mesh created with libfive.
  This example will present an input dialog—the value you type will be used as the sphere radius (to limit the size of the generated STL file recommended test value is 2 or less).
  Click the "Cancel" button the second time the dialog appears (which signifies "end of input"). [Hey, this is only a Proof-of-Concept, ok...]
  (Note: This PoC doesn't actually create bindings to libfive, it just retrieves the generated .stl file (produced by the C++ example) from the Emscripten/WebAssembly "virtual filesytem". A True Port would use bindings to the libfive library.)

(See also: https://github.com/Omniblox/libfive/tree/add-web-assembly-support)