Recursive vs. One-shot Parser to fix #3

[bhavyakukkar]

implements a fix for #3

• new switch parse_defs on struct Parser that is read in Parser.read_one and disables parsing new syntax definitions if false
• made Parser public so that it can be used directly without relying on fn parse
• made field Parser.reader public so that it can be used as it was being used for reporting location of failure in fn parse
• new Parser constructors Parser::recursive and Parser::one_shot to construct Parsers that do and do not have the ability to parse new syntax definitions, respectively
• made fn Parser.read_all public so that it can be used directly without relying on fn parse

[bhavyakukkar]

sorry for the force-push (synced with new changes in main by mistake)