[plt-scheme] MysterX in v204

The version of MysterX released with PLT Scheme v204 
has a rather large change, which was not mentioned in the 
release announcement.  When possible, it uses 
direct COM interfaces rather than calling IDispatch::Invoke 
(COM Automation).  When making a direct method call, MysterX 
extracts a C function pointer to the method entry point, sets 
up a stack frame, and calls through the function pointer.

Existing MysterX code should work without change.

The code to make all this happen involves inline assembly, 
and is rather delicate (which is why I hadn't done it 
until now).  As usual for COM, the Microsoft documentation 
is fairly opaque, and one often has to guess how things are 
really supposed to work.  Unfortunately, the current MysterX 
test suite is not exhaustive.  While I've tested the code with a 
number of COM servers, one should be on the lookout for any 
problems.  

The benefit of using direct calls is speed.  For in-process 
COM servers, method calls seem to be about twice as fast.  For 
out-of-process servers, there is still some benefit.  For 
an Excel method I tried, calls took about 20-25% less time.

-- Paul