Add foreign functions and rename types

* src/assimp.scm: Rename types, from scene to ai-scene, from node to
  ai-node, etc. New functions ai-import-file and ai-multiply-matrix4.

* src/low-level.scm: New syntax define-foreign-function for simplifying
  foreign functions declaration.

* src/low-level/cimport.scm: Foreign functions aiImportFile and
  aiMultiplyMatrix4.