Use plain uint array for InversionList (std.uni)

Storage for static data was long since compressed so it no longer plays a role at compile-time.
Memory requirements at run-time are modest anyway.

Benchmark:
```d
import std.uni, std.datetime, std.stdio;

void main()
{
    StopWatch sw;
    size_t count;
    CodepointSet set = unicode.L;
    sw.start();
    foreach(i; 0..10)
    foreach(u; dchar.min..dchar.max+1U)
        if(set[u])
            count++;
    sw.stop();
    writeln("Total: ", count, " in ", sw.peek().msecs, " msec");
}
```

Some numbers on Win32:
```
C:\dmd2\src\phobos>bench_set_old
Total: 1010130 in 1106 msec

C:\dmd2\src\phobos>bench_set
Total: 1010130 in 857 msec
```