bugfix: parse_file returns helmold::value, not std::string
include helmold headers in main.cpp
add '_' prefix to a member variable
place '&'s and '*'s just after type names, not just before variable names
move char predicates to lexer class
rename token::kind to token::kind_t
resolve name conflict between token class and lexer#token
move push_error method
add a comment mentioning a todo
introudce token_p type (alias for shared_ptr<token>)
wip: add lexer#set_token
resolve name conflict between method and type