func Reverse(*Template) *Template
returns a template that produces the reverse of the original
for any input.

Changes outside exp/template/html include:
- Adding a getter for a template's FuncMap so that derived templates
  can inherit function definitions.
- Exported one node factory function, newIdentifier.
  Deriving tempaltes requires constructing new nodes, but I didn't
  export all of them because I think shallow copy functions might
  be more useful for this kind of work.
- Bugfix: Template's Name() method ignores the name field so
  template.New("foo") is a nil dereference instead of "foo".

Caveats: Reverse is a toy.  It is not UTF-8 safe, and does not
preserve order of calls to funcs in FuncMap.

For context, see http://groups.google.com/group/golang-nuts/browse_thread/thread/e8bc7c771aae3f20/b1ac41dc6f609b6e?lnk=gst

R=rsc, r, nigeltao, r
CC=golang-dev
https://golang.org/cl/4808089