A lot of functionality added

* src/assimp.scm: New foreign functions mapped:
  ai-get-material-color
  ai-get-material-float-array
  ai-get-material-integer-array
  ai-multiply-matrix3
  ai-identity-matrix3
  ai-identity-matrix4
  ai-transpose-matrix3
  ai-transpose-matrix4

* src/low-level.scm: New tools for C library support:
  define-assimp-function
  define-enumeration
  define-bitfield

* src/low-level/cimport.scm: New foreign functions defined:
  aiMultiplyMatrix3
  aiMultiplyMatrix4
  aiIdentityMatrix3
  aiIdentityMatrix4
  aiTransposeMatrix3
  aiTransposeMatrix4

* src/low-level/material.scm:
  Parsers: parse-aiMaterial, parse-aiMaterialProperty
  Enumerations: ai-material-key
  Functions: aiGetMaterialColor, aiGetMaterialFloatArray, aiGetMaterialIntegerArray

* src/low-level/postprocess.scm: Flags used when importing assets.