fun project - improving calcHash

[Walter Bright]

https://github.com/D-Programming-Language/dmd/blob/master/src/root/stringtable.c#L21

Profiling shows the calcHash function is a significant contributor to 
compilation time (3.25% of total time). So making it faster is a win. Even 
making dmd 1% faster would be a nice win - all those little drops add up.

There are many, many string hash functions findable through google. Anyone want 
to spend the effort to make a faster one? Remember, ya gotta prove it's faster!

A nice timing test would be the time expending compiling Phobos. I would suggest 
that the 64 bit build of dmd be used for timing tests.

Also, be careful, many of those hash functions on the intarnets have a license 
that makes it unusable for dmd.