Fix Issue #12877: allow uniform() to handle dchar variates

Since [dchar.min, dchar.max] does not cover the full bit range of dchar,
optimizations that work for other integral types will fail here, and
result in illegal unicode points being generated.

This fix avoids breaking any existing calls to uniform() via a twofold
approach:

   * uniform!"[]"(T.min, T.max) will only call uniform!ResultType if
     T is not a dchar;

   * uniform!dchar will not try and use the entire bit range but will
     instead call uniform!"[]"(dchar.min, dchar.max).

Unittests have been added that should prevent such issues from arising
again.