Feature/inline help

This function lists  is functions and types provided by a module

Mandatory modules are load without the need to be explicitly imported
in the script file.

By now the only required module is 'core'.

This is a special module that is *always* imported

For this first version of the module we have three functions:

getAvailableModules() -> string
which returns the list of available pugs modules

getPugsVersion() -> string
getPugsBuildInfo() -> string
which are moved from the 'utils' module

There is no need to use `import core` in the preamble of a pugs script
file.

The purpose of this is to provide some global execution info to parts
of execution.

This defines a singleton that is only constructable in the `parse`
function. The instance thus only leave during the execution of the
script.

As a first stored, the ugly `p_root_node` static pointer has been
encapsulated in this container.