Adding DisplayStr impl for &dyn DisplayStr

[luketpeterson]

This is related to #32

The following code would work if &dyn DisplayStr implemented DisplayStr

    let template = "{body} = {some_number:2.4}";

    let mut data: HashMap<String, &dyn DisplayStr> = HashMap::new();
    data.insert("body".to_string(), &"This is a string");
    data.insert("some_number".to_string(), &5.0);

    let output = strfmt(template, &data)?;

A HashMap of &dyn DisplayStr is more efficient than the current best solution for heterogeneous input types, which is Box, because each box requires a separate allocation.

Thank you.

[vitiral]

This needs tests. Does the example code actually run with just this change? It's been years since I seriously wrote rust, if this is all it takes then dyn is cooler than I realized.

Also, why &"This is a string"? Why is the &? Is that some kind of signal to the compiler to make it dyn?

[luketpeterson]

I pushed a new test to the branch. test_heterogenous_values

What's happening is this:

• "This is a string" has type &str
• There was already an implementation for DisplayStr for &str
• I could only add an &str as a value to a HashMap<_, &str>, but then I couldn't add an integer to that same map
• &"This is a string" has type &&str, which can be cast to &dyn DisplayStr because &str implements DisplayStr
• Likewise & f64 can also become &dyn DisplayStr, so they can coexist in the same HashMap