refactor PolicyExpr to use Visitor Pattern

[j-lanson]

Resolves #457 .

This is also a portion of the proposed changes in #456 RFD8 .

1. Updates some Expr variants to have their own proper structs so we can have visit_X patterns on them
2. Add Into<Expr> for each Expr sub-type to make it easier to turn a proper struct into its Expr variant
3. Added ExprVisitor pattern
4. Implemented ExprVisitor to perform policy expr execution, replaced pub(crate) eval() func

Currently ExprVisitor is implemented on Env<'_>, but as is mentioned in the RFD, we might want other "visitors" on an Expr tree. In a subsequent PR as it becomes relevant, I recommend changing it so Executor implments ExprVisitor and the env.rs functions like binary_primitive_op are changed to take a &Executor. This way, additional structs that take an Env can be created to impl ExprVisitor to do different behavior.