Add default value for properties

[AlephAlpha]

Fix #775

We will be able to provide default values to properties, using a syntax like:

#[derive(Properties)]
struct MyProps {
    #[props(default = "50 + 50")]
    value: u32,
    #[props(default = "true")]
    enabled: bool
}

The syntax is based on @mdtusz's suggestion, but it takes an expression instead of a path to a function.

[AlephAlpha]

Sorry, I forgot to change the function name when copying and pasting. It should be something like default_value_should_work.

[mdtusz]

Nice work! I'm not sure how I feel about the string expressions though. It seems like it would quickly become not-so-nice for non-primitives, and might invite "bad" usage patterns.

Part of my thinking behind the string-to-function approach was just to somewhat mirror how serde works.

Thoughts @jstarry @hgzimmerman?

[hgzimmerman]

I personally would prefer if the default value was specified by a path/identifier to a no-argument function that returns the expected type. I do think that this is a good start though!

[jstarry]

I think we can close this, I also prefer #881