Update macro rules to account for 1.32.0 changes

[TonySamuels]

https://blog.rust-lang.org/2019/01/17/Rust-1.32.0.html

Macro improvements
A few improvements to macros have landed in Rust 1.32.0. First, a new literal matcher was added:

macro_rules! m {
    ($lt:literal) => {};
}

fn main() {
    m!("some string literal");
}

literal matches against literals of any type; string literals, numeric literals, char literals.

In the 2018 edition, macro_rules macros can also use ?, like this:

macro_rules! bar {
    ($(a)?) => {}
}

The ? will match zero or one repetitions of the pattern, similar to the already-existing * for "zero or more" and + for "one or more."

[TonySamuels]

This resolves #27.