The verifier reports the 'location' of an error message as a reference
to the entity that has a problem. This uses the 'AnyEntity' type to
refer to instructions/values/ebbs etc.
Also add an err! macro similar to the one used by the parser.
Export a single function: parse_functions() which results a vector of
functions parsed from the source string.
Hide the parser and lexer modules. They are not useful to external
clients.