Tweak unicode Trie generation speed

Turns out 2 optimizations were broken and never got us much of any speed.
20 ms vs 400us, so around of 50x speed up.
This is crucial to my efforts on putting std.regex on new rails of std.uni.

Test program:
```d
void main()
{
    alias String = string;
    import std.regex, std.uni, std.stdio;
    import std.datetime;
    import std.internal.uni_tab, std.internal.uni;
    StopWatch sw;
    sw.start();
    auto ctPat = unicode.Nd.codepointSetTrie!(13,8);
    sw.stop();
    writefln("%s us", sw.peek().usecs);
    sw.reset();
    sw.start();
    auto ctPat2 = std.internal.uni.CodepointTrie!8(unicodeNd);
    sw.stop();
    writefln("%s us", sw.peek().usecs);
}
```

Anyhow before this pull I see:
```
~25 000 us
~45us
```
After this pull:
```
~440us
~45us
```