$//$ Project $0$: Structs and Traits in Rust, Parsing$/$ASTs$,$ S$-$expressions, Polish notation

$//$ Write a parser that parses an S$-$expression language for Polish notation and converts it into an AST using the structs defined in the lib.rs file

$//$ The language should support $4$ artihmetic operations: $+,-,*,^$ where arguments can be integers or other polish notation expressions

$//$ Parentheses are optional for binary operators, but are mandator in other k$-$ary instances

$//$ These are some examples of equivalent inputs:

$//+125=>(+125)$

$//-+321=>(-(+32)1)$

$//(-321)=>(-(-32)1)$ Note that operations $+,-,$ and $*$ are all left associative

$//(^321)=>(^3(^21))$ Note that the operation $^$ is right associative

$//(-2)=>(-02)$

$//(+3)=>3$

$//$ Note that $+$ and $-$ support unary arguments, whereas $*$ and $^$ do not

$//$ Parse errors: return an ErrorExp struct

$//$ e$.$g$.,$

$//(*4)$ return ErrorExp, $*$ and $^$ cannot be used as unary operators

$//(^5)$ ditto

$//()$ return ErrorExp, parens cannot go alone

$//(+)$ return ErrorExp, operators must have $1$ or more operands in all cases

$//$ x return ErrorExp, only numeric $0-9$ characters, whitespace, $(,),+,-,*,^,$ are valid input characters