More functions from original library

* src/assimp.scm:
  New functions: ai-release-import
                 ai-attach-predefined-log-stream
                 ai-detach-all-log-streams
  Bug resolved with size of vertices structs when wrapping meshes.

* src/low-level/cimport.scm:
  New foreign function supported:
      aiReleaseImport
      aiGetPredefinedLogStream
      aiAttachLogStream
      aiDetachAllLogStreams

* src/low-level/types.scm: New enumeration ai-default-log-stream