Performance improvements for D / DMD compiler.

[Dave]

What's on your wish-list (let's leave the GC and other library functionality out of this for now <g>)?

Here's mine:
   - Better floating point code generation
   - Put the tail-recursion optimization back in
   - Omit frame pointers w/ -O switch
   - Improved in-lining (don't arbitrarily exclude functions w/ inout parameters and loops).
   - Auto-finalization.