Refactor builders to be const

[Matt3o12]

Hello thank you for your work.

I have made changes so that all builders are now const. They should still be 100% backwards compadible. I had to introduce a constructor, new, as Default cannot be const just yet.

This allows me to define a const builder at the top and use it like this:

    const BACKOFF_BUILDER: ExponentialBuilder = ExponentialBuilder::new().with_min_delay(Duration::from_millis(1));

    fn test() {
        my_fn().retry(BACKOFF_BUILDER).call();
    }

All changes:

• Refactored all methods inside FibonacciBuilder, ExponentialBuilder, and ConstantBuilder to be const.
• Added a new const new constructor for each builder which uses the values from default()
• Updated the default method to call new().
• Added unit tests for the const builders to ensure they have the correct values.

[Matt3o12]

It seems like debug_asserts are causing problems with the MSRV:

    /// Set the factor for the backoff.
    ///
    /// # Panics
    ///
    /// This function will panic if the input factor is less than `1.0`.
    pub const fn with_factor(mut self, factor: f32) -> Self {
        debug_assert!(factor >= 1.0, "invalid factor that lower than 1");

        self.factor = factor;
        self
    }

When do you plan to bump the MSRV or do you have an idea how to fix that in the meantime? :)

[Xuanwo]

Hi @Matt3o12, thank you for your work first! I believe we can remove that debug_assert. Would you like to update the documentation for with_factor to emphasize the importance of ensuring that factor >= 1?