#33 Use std::valarray for increased processing performance

[Rowan James]

The std::valarray library is a standardized implementation of vectorized arithmetic operations, ideal for simple tile-based operations as seen in Parbat.

Implementations can take advantage of specialized SIMD instruction sets in modern processors.

Perhaps a free MMX/3DNOW[EX]/SSE[2,3] valarray implementation is available, or would be a good idea for a seperate project...