Expression->String->Expression doesn't always work

[genos]

In working on #210, I wanted to serialize a static collection of expressions to simplify for consistent benchmarking. Unfortunately, Expression::from_str(&expression.to_string()) doesn't reliably work.

For instance, this test:

proptest! {
    #[test]
    fn round_trip(e in arb_expr()) {
        let s = e.to_string();
        let p = Expression::from_str(&s);
        dbg!((&e, &s, &p));
        prop_assert!(p.is_ok());
        prop_assert_eq!(p.unwrap().into_simplified(), e.into_simplified());
    }
}

finds the following failure:

[quil-rs/src/expression.rs:1513] (&e, &s, &p) = (
    Infix(
        InfixExpression {
            left: Address(
                MemoryReference {
                    name: "A0",
                    index: 0,
                },
            ),
            operator: Caret,
            right: Address(
                MemoryReference {
                    name: "aa",
                    index: 0,
                },
            ),
        },
    ),
    "A0[0]^aa[0]",
    Err(
        Syntax(
            Leftover(
                LeftoverError {
                    line: 1,
                    column: 6,
                    snippet: "OPERATOR(^)",
                    parsed: Address(
                        MemoryReference {
                            name: "A0",
                            index: 0,
                        },
                    ),
                },
            ),
        ),
    ),
)
thread 'expression::tests::round_trip' panicked at 'Test failed: assertion failed: p.is_ok() at quil-rs/src/expression.rs:1514.
minimal failing input: e = Infix(
    InfixExpression {
        left: Address(
            MemoryReference {
                name: "A0",
                index: 0,
            },
        ),
        operator: Caret,
        right: Address(
            MemoryReference {
                name: "aa",
                index: 0,
            },
        ),
    },
)
        successes: 0
        local rejects: 0
        global rejects: 0
', quil-rs/src/expression.rs:1336:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note that this test relies on changes made to arb_expr() and friends in #210.

[genos]

Closed in #210, using the parenthesized version to ensure consistency.