Add Default trait impl for Lazy to enable struct field defaults

[dvaergiller]

Implement Default for Lazy<fn(&mut Buffer), C> to support using #[derive(Default)] on structs containing Lazy fields (e.g., for component children).

The default value creates an empty Lazy that renders nothing.

[dvaergiller]

#166

[adamjhf]

Thanks! I hit this too.

[vidhanio]

I am working on a better idea for this, specifically making any Lazy<F> optional. I think it is awkward that only fn supports this.

[dvaergiller]

Yeah I agree, I had a go trying to implement it for Lazy<F> but I could not give a default value when the type was not fully known so I thought it could not be done.

If it can, then yes that would be better.