std.uni - speed up Trie construction by factor of ~3

Take advantage of word-at-once checking for slices of bit-packed arrays in the trie.

Now it's seems to be either on par or much faster (depending on the set) then the venerable std.regex internal trie.


Tiny test:
```d
import std.uni;

void main()
{
    import std.regex, std.uni, std.stdio;
    import std.datetime, core.memory;
    import std.internal.uni_tab, std.internal.uni;
    StopWatch sw;
    GC.disable();
    sw.start();
    foreach(_; 0..1_000)
        auto set = unicode.Nd;  
    sw.stop();
    writefln("Find set - %s us", sw.peek().usecs);
    
    auto set = unicode.Nd;
    sw.reset();
    sw.start();
    foreach(_; 0..1_000)
        auto ct1 = set.codepointSetTrie!(10, 11);
    sw.stop();
    writefln("Build Trie - %s us", sw.peek().usecs);

    sw.reset();
    sw.start();
    foreach(_; 0..1_000)
        auto ct2 = std.internal.uni.CodepointTrie!8(unicodeNd);
    sw.stop();
    writefln("Build Trie old - %s us", sw.peek().usecs);
}
```