Move some of svn_types.h into svn_interfaces.h

Hi all,

I've been trying to determine a way to consolidate the duplicate types
of svn_repos_file_rev_handler_t and svn_ra_file_rev_handler_t types into
the same definition. Much like svn_log_message_receiver2_t, the file
rev handlers are used by multiple libraries to pass data between them,
and have identical definitions.

My first attempt was to define svn_file_rev_handler_t in svn_types.h and
implement the library specific versions as typedefs. That didn't work
due to circular dependency issues with svn_delta.h and the

So, here's my next attempt: creating a new header file,
svn_interfaces.h, and putting the definition of svn_file_rev_handler_t
in it. I've also moved svn_log_message_receiver2_t, and the prototype
for its compatibility wrapper to the new header file, and updated callers.

Are there any objections to this approach, or suggestions for a better
one? Also, what are the backward compatibility implications of moving
other interface definitions in svn_types.h to svn_interfaces.h?

-Hyrum