Expose DerivationPath methods

[ItoroD]

Describe the enhancement

DerivationPath is a type in key.rs. Only its initialization method has been exposed, there are more useful methods that should be exposed on this.

Additional context

With DerivationPath having more useful methods, we can use/update places where a different type was used in place of DerivationPath.

[ItoroD]

@thunderbiscuit I hope I was able to capture the issue well. You might have more context you want to add?

[thunderbiscuit]

Yep this is roughly what I had in mind! One place we can use it is here on the Psbt type.