Added Register (for int64) type.

[ldemailly]

• [x] Rename object.Value() ObjValue() so an object can be both ast.Node and ast.Object
• [x] simple loop (see loop.gr) shows great improvement (no reg is 50% slower)
• [x] added register for integer function arguments
• [x] more benchmarks (mandelbrot.gr shows no noticeable improvements somehow)
• [x] more profiles
• [ ] extend to other types