Split #168 into two PRs as
requested, this is the one for no_std support.

---------

Co-authored-by: Xuanwo <github@xuanwo.io>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

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.

---------

Co-authored-by: matt3o12 <1218474+Matt3o12@users.noreply.github.com>

Signed-off-by: Xuanwo <github@xuanwo.io>

Signed-off-by: Xuanwo <github@xuanwo.io>