Adjusts mpl::pair concept to be compatible with STL pairs

[t_schwinger]

WHAT:
=====

The patch changes the MPL pair concept, so STL pairs
and Fusion pairs also work as MPL pairs.

The old type members have been added after testing for
backward compatiblity with 3rd party code.


STL pair:  first       (value)
 second      (value)
 first_type  (type)
 second_type (type)

Fusion pair:
 second      (value)
 first_type  (type)
 second_type (type)

MPL pair (CVS):
 first       (type)
 second      (type)

MPL pair (after applying this patch):
 first_type  (type)
 second_type (type)


AFFECTED LIBRARIES:
===================

- MPL (numerous files, code and documentation)
- Variant (2 files)
- Parameter (1 file)


FILES:
======

- boost/mpl.patch 
- boost/parameter.patch 
- boost/variant.patch 
- libs/mpl.patch 


TESTED WITH:
============

- MSVC 8.0
- MSVC 7.1
- GCC 3.4.2 

[Aleksey Gurtovoy]

I guess I don't fully understand the motivation for this one. MPL pair is mostly an auxiliary structure of secondary importance; the only place is the library that dictates usage of the MPL pair protocol is mpl::map. Other than that, using MPL to manipulate STL pairs and/or Fusion pairs should not be any harder than using it to manipulate MPL pairs.

Could you elaborate on the specific use case that triggered this patch?

[Steven Watanabe]

Without further information from the reporter, there's nothing doing.