Add a LISP-style "let" statement

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Sometimes it would be nice to have an explicit way of assigning the value of 
one name to another so that, in situations where there is a name collision, the 
template writer could be specific about choosing which name they wanted rather 
than not have access to names higher up in the scope chain.  A good example of 
such a collision is detailed in issue #52 with the name "url".

I think a "let" statement, such as LISP's, would work well here.

For example, here's the template and expansion dictionary from issue #52 and 
the documentation, with a "let" statement added.

{.section songs}
  <h2>Songs in '{playlist-name}'</h2>
  {.let playlist-url url}
    <table width="100%">
    {.repeated section @}
      <tr>
        <td><a href="{url-base|htmltag}{url|htmltag}">Play</a>
        <td><i>{title}</i></td>
        <td>{artist}</td>
        <td><a href="{url-base|htmltag}{playlist-url|htmltag}">Link to playlist</a>
      </tr>
    {.end}
    </table>
  {.end}
{.or}
  <p><em>(No page content matches)</em></p>
{.end}

... combined with a data dictionary ...

{
  "url": "happy!",
  "url-base": "http://example.com/music/", 
  "playlist-name": "Epic Playlist", 
  "songs": [
    {
      "artist": "Grayceon", 
      "title": "Sounds Like Thunder"
    }, 
    {
      "url": "2.mp3", 
      "artist": "Thou", 
      "title": "Their Hooves Carve Craters in the Earth"
    }
  ]
}

Perhaps it is not the best example to illustrate the usefulness of this 
language addition, but hopefully it is clear that it does solve an actual 
problem.

Original issue reported on code.google.com by elij...@gmail.com on 5 Nov 2010 at 2:08