Make variables mutable

[kevaundray]

This entails

• adding an AST and modifying the parsing grammar
• modifying the analysis phase to check if a variable is mutable, if it has been assigned
• modify the evaluator to interpret the assign statement

[kevaundray]

The previous way to do it is here: https://github.com/noir-lang/noir/commit/03e640285228d4654e7e8b1c8d3e48502724cabd

However, this method means we need to have both an AssignExpression and an AssignStatement. What we could do it, to check for an identifier in the parse_statement method. We would need to peek ahead at the next token to see if it is an assign operator, because we could also have an identifier/expression which starts with an identifier