The v1.0 release of serde-env

[Xuanwo]

Serde-env has been used in production for a while, and I believe it's ready for wider adoption. It's time for us to clean up some existing obstacles in this project, establish the scope, and deliver on our promises.

Do you have any ideas?

cc all our contributors: @Pscheidl, @jayvdb, @kgtkr, @Tony-Samuels, @acanthite1855, @wyatt-herkamp

[Pscheidl]

1. Resolve all todos in the codebase.

2. All seeds features should be supported. Especially "flattening". There's a 2 year old issue for that.