Separate CPM binary files from CPM source.

CPM binary files (as generated by CMake and make) are now separated from
the the downloaded source. As such, you can now separate the CPM bin
directory from the CPM source directory. Useful for making quick changes
to a CPM module's source (and cleaning without deleting the source you
changed).

No need to fiddle with CPM_ROOT_BIN_DIR; just change CPM_DIR to point to
your new source directory in your project's setup. The CPM binaries will
remain under your binary directory as cpm-bin.
Add tests for source/bin separation.