text/template: map key sorting should know about the sort interface

[robpike]

Now that reflect.SliceOf exists, it's conceivable that key sorting for maps in
text.Template could be craftier: if the a slice of key types is sortable, it could
promote to a sortable slice and sort them.

The user can do this by hand already, of course, but it might be nice to be automatic
about it. Low priority.

[rsc]

Comment 1:

The slice of key types K is []K, which has no methods and so cannot implement
sort.Interface.

Status changed to Invalid.