Draft: Add compartmentalized global `camr` object with methods to manage paths and supplement `goto`.

Optional whether we merge this as I believe we are phasing out goto. These may be helpful in non-targets projects, however.

Several functions have been added to compartmentalize global variables and add path safety.

• camr_cache creates a global list called camr whose elements may be modified through the camr functions only.
• camr_save saves this object to disk in a file called .cache.
• camr_reset resets the list.
• camr_pushd and camr_popd operate as the shell commands pushd and popd, allowing directory paths to be saved to and restored from a stack.

The camr origin is the initial working directory in which camr_cache is called (the project directory ideally).

The camr prefix is the directory against which relative paths are specified. For example a Dropbox folder or the origin.

• camr_prefix defines the prefix.
• camr_paths creates the usual output directories under the prefix path.