Add support for changing a method's IL at runtime.

These new functions provide a feature similar to one that is available
through Microsoft's .NET Profiling API.

With the new functions, a profiler can modify the implementation of a
method to add instrumentation or for other purposes.

Functions added:

mono_method_get_il_body (public)
  Returns a pointer to the IL of a method. Works similarly to
  mono_method_get_header, but doesn't do any processing on the bytes.

mono_profiler_replace_method_body (public)
  Allows a profiler to replace the IL of a method.

mono_profiler_get_method_replacement (internal)
  Retrieves the replacement body for a method, if one exists.