[postcard-schema] Adding support for uom 0.37 in impl/

[PedromcaMartins]

I was trying to use postcard to send Uom (https://crates.io/crates/uom) and nalgebra structs over the wire, but it seems that postcard-schema doesn't implement a schema for uom.
I'll go ahead, and fork the project to implement this feature.
I greatly appreciate your feedback, as this is my first Issue ever.
And also, let me know if you're also interested in creating a schema for uom.

Cheers ;D

[jamesmunns]

@PedromcaMartins please feel free to open a PR to add support for uom to postcard-schema, and let me know if you have any questions along the way!