Adds serde_derive feature

[MathisWellmann]

to allow serialization and deserialization of the Decimal struct.
It only takes effect if the dependency is specified as such:

fpdec = { version = "*", features = ["serde_derive"] }

The reason for this addition is that I would like to be able to rely on serde support in lfest to serialize the BaseCurrency and QuoteCurrency among others.
I would love to see this change get merged and published so a new version of lfest can be published as well.

Thank you for such an amazing crate, I'm glad I found it!

[mamrhein]

Thx for the PR.
Unfortunately, Decimal is using an i128 integer for the coefficient and serde doesn't seem to support integers larger than 64-bit out of the box.
I'll have to dig into serde to find a solution.

[MathisWellmann]

After some re-thinking, I will not directly serialize or deserialize the Decimal type as its rather confusing from a user-facing perspective to have to specify the inner fields. Consider this PR low priority or feel free to close it as well.
Cheers!

[mamrhein]

I've created a branch 'serde'.
The feature 'serde_as_str' allows Decimal instances to be serialized to / deserialized from String.
Maybe you want to give it a try.
If so, please let me know what you think about it.

[MathisWellmann]

Just tried out your new feature branch and it looks excellent.
Using the From<Decimal> and TryFrom<String> trait implementations through the serde features is a fantastic way, especially from the users perspective (As they should not be bothered to specify the coeff).
Having to enable both serde and serde_as_str features is the right choice as well, as not everyone will want to use String as the intermediate representation.
Integrating the feature is straightforward as well.
I certainly would be happy seeing these changes in a crates.io release.
Gute Arbeit :)

[mamrhein]

Solved by adding feature 'serde-as-str'. See version 0.6.0.