monkey interpreter in rust following the book https://interpreterbook.com/

made some additions and modifications from the book:

• added ||, &&, // and % operators
• reassigment of variables
• exit and env repl commands
• semicolons are significant: semicolon = statement, statements = none. No semicolon = expression, expressions = a value
• blocks ({}) are expressions, they evaluate to the last expression in the block (or a return statement if there is one), however they do not introduce a new scope
• added standard library of code that is loaded at the start of the repl (map, filter, reduce, sum)
• type builtin function to get the type of a value
• probably more stuff I forgot