[RFC] Introduction of a namespace for CBMC

[esteffin]

As described in the coding standards most of CBMC code lives outside of a namespace.

However having a namespace can be beneficial in some cases, one of which is the libcprover-cpp library as a user may not want to pollute his application object space with unqualified symbols coming from CBMC.

This RFC aims to open a discussion about whether we should:

1. continue with a no-namespace coding style,
2. introduce a namespace only for the library and some other components (where beneficial),
3. introduce a namespace for the whole of CBMC.

If the decision is to introduce a namespace for CBMC, which one will it be?