Use strdisplaywidth where available

In vim 7.3, the strdisplaywidth function was added to give an accurate
count of the number of screen columns that will be taken up by a given
string.  Tabular was written before vim 7.3, and was implementing this
functionality itself (at least for tabs; it wasn't trying to handle
doublewide characters).

This changeset allows us to take advantage of strdisplaywidth where it
exists.

autoload/tabular.vim

@@ -46,7 +46,14 @@ set cpo&vim
 
 " Return the number of bytes in a string after expanding tabs to spaces.  {{{2
 " This expansion is done based on the current value of 'tabstop'
+if exists('*strdisplaywidth')
+  " Needs vim 7.3
+  let s:Strlen = function("strdisplaywidth")
+else
   function! s:Strlen(string)
+    " Implement the tab handling part of strdisplaywidth for vim 7.2 and
+    " earlier - not much that can be done about handling doublewidth
+    " characters.
     let rv = 0
     let i = 0
 
@@ -62,6 +69,7 @@ function! s:Strlen(string)
 
     return rv
   endfunction
+endif
 
 " Align a string within a field                                           {{{2
 " These functions do not trim leading and trailing spaces.