Revised project goals.
Continued revising node subtypes.

- Re-organized typedefs.
- Lamda's now take list of nodes to be evaluated rather than single expression.
- plisp_function_call_t was renamed plisp_c_function_call_t.
Continued revising node subtypes.

- Removed VOID subtype, as it will be represented by plain C NULL.
- Added FORK subtype, which will be used to facilitate was formerly called shadow values.
- Improved documentation.