bsls_byteorderutil template dispatch

Looking into issue 125, it started to look like trying to deal with questions like the width of `wchar_t` by maintaining a knowledgebase of platform/os/configuration combinations was just not the way to go in the long term.

This pull request presents a possible template-based dispatch mechanism to connect the 'bsls_byteorderutil' interface with the concrete implementations in 'bsls_byteorderutil_impl'.

I'm presenting this first for discussion.  There is still work to be done.  In particular, I have not added direct tests of the dispatch mechanism.
If these are superfluous in `gcc`, `clang`, etc. shouldn't they be inside:

```
#if defined(BSLS_PLATFORM_CMP_IBM) || defined(BSLS_PLATFORM_CMP_SUN)
```

?