Speed up \tl_if_single:nTF by avoiding string comparison

At the cost of introducing one auxiliary, the function is
now 5x faster than before in LuaTeX, and about 1.5x in
other engines.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5031 de43f980-851b-0410-b2f7-c40aca1f87e0