Separate R runtime from the server executable

The first commit is the interesting one:

> We discussed this with @hmansell and this is probably the most reasonable setup.
The original code worked, but had an unfortunate consequence that the generated
code and the type provider referenced an assembly compiled as executable - which
is confusing. So, this splits the R provider into 4 assemblies - I'll add more
documentation in the next commit.

The second commit just updates documentation and adds an explanation for the structure.