Geometry validation

Should we implement the geometry validation within the [rust-geo](https://github.com/georust/rust-geo) or in an external dedicated crate?
I have the feeling that, thanks to cargo, in the rust community its preferred to split in a separate crate.
What do you think?

It would be nice to be able to:
- trait for each geometry type to check if a geometry is valid or not
- define functions to fix common errors and problems.

No files, tests, or entry points are named. Start by reviewing the issue's question about placing geometry validation in rust-geo or an external dedicated crate, then define the validation traits and common-error fixes that would be in scope; done means the crate boundary and initial feature scope are agreed.

Written by the indexing model from the issue text.