Very simple SIMD programming

[jerro]

> Simple example:
>   T opCompound(string seq)(T a, T b, T c) if(seq == "* +") { 
> return
> _madd(a, b, c); }

It may be useful to have a way to define compound operators for 
other things (although you can already write expression 
templates), but this is an optimization that the compiler back 
end can do. If you compile this code:

float4 foo(float4 a, float4 b, float4 c){ return a * b + c; }

With gdc with flags -O2 -fma, you get:

0000000000000000 <_D3tmp3fooFNhG4fNhG4fNhG4fZNhG4f>:
    0:   c4 e2 69 98 c1          vfmadd132ps xmm0,xmm2,xmm1
    5:   c3                      ret