This allows a query to use the string length in a Solr query, without having to index a separate field with the length. The function is named "strlen" in the patch, but another option is "stringlength".
Since this also introduced a new ValueSource in Lucene, I'm not sure if I should split the patch up into two separate issues, but as the Solr patch would rely on the Lucene patch being accepted, I've included them both in this issue for now.
Patch includes two tests in TestFunctionQuery. Patch is against the current HEAD from github/lucene-solr.