Implement a KiCad component classifier

[luxas]

Commits:

• Prep work: move DynamicResult to error.rs, and improve marshal_yaml to write to a writer, not a string
• Refuse to unmarshal YAML if there are unknown fields (I spent quite some time with a typo in the YAML :sweat_smile:). Default optional fields so they can be unmarshalled without errors.
• Make it possible to annotate a component with a "generic" label, for potential use in a unix pipe by plugins
• Add a Labels trait similar to https://pkg.go.dev/k8s.io/apimachinery@v0.21.1/pkg/labels#Labels
• Add a Requirement enum similar to https://pkg.go.dev/k8s.io/apimachinery@v0.21.1/pkg/labels#NewRequirement
• Implement a classifier (as a separate binary) for components that assigns a class to components given other attributes

The sample data I used for policy.yaml is:

classifiers:
- class: capacitor
  labels:
  - key: symbolName
    op: Equals
    values:
    - "C_Small"
- class: capacitor
  labels:
  - key: footprintLibrary
    op: Equals
    values:
    - Capacitor_SMD
- class: resistor
  labels:
  - key: symbolName
    op: Equals
    values:
    - R_Small
- class: shunt_resistor
  labels:
  - key: symbolName
    op: Equals
    values:
    - R_Small
  attributes:
  - key: Tolerance
    op: Equals
    values:
    - "1"

I used the Kubernetes API way of matching labels in order to be sure it's at least somewhat generic and battle-tested, instead of inventing my own thing. If you go and check the Go code implementing this in k8s, and compare to this implementation, one can see that Rust really shines for this task.

@chiplet @twelho