Dynamic C libs, interpreter IO, :x command, start of type providers

New features:
 * Language extension mechanism + extension for type providers, disabled by default
 * Beginning of type provider implementation (works for simple case)
 * Beginning of interpreter IO (io_bind still missing)
 * Dynamic loading of C libs
 * FFI calls from interpreter

New user-facing features (:dynamic, :x, %dynamic) have a test case and are added to the tutorial. The test passes on Linux, I don't have a Windows machine handy.

Portability to Windows may be compromised, but all the Unix-specific stuff is hidden away in Util.DynamicLinker, so it should be straightforward to add conditional compilation if Cygwin doesn't take care of it.