Introduce cmState class.

At this point, it is an interface to the cache.  It will be extended
to be a universal interface for access to and manipulation of
configuration-time data (defintions, properties on targets,
directories, source files etc).

This will allow porting all command implementations away
from the cmMakefile and cmTarget classes, and result in something
more-purely related to configuration-time processing of cmake
commands.  That should serve at least the following goals:

 * Split the CMake implementation more definitively into three
   stages: Configuration, computation and generation, and be able to
   implement each optimally for memory access patterns etc.
 * Make better IDE integration possible by making more configuration
   data available.
 * Make it possiblte to use a smaller library than CMakeLib.a in
   cpack and ctest, resulting in smaller executables.
 * Make it possible to run the configure step multiple times in
   the same CMake run (#14539).

Manage its lifetime in the cmake class, and add a convenience accessor
to cmMakefile.