Import Prefix direct from provider nkeys.ts instead of mod.ts

[cmidgley]

The file curve.ts imports Prefix from mod.ts, which imports it from nkeys.ts. This PR changes the import to go directly to nkeys.ts.

Background:

I am working on a project to get the client to work on a platform called Moddable, a modern JS engine (called XS) that targets microcontrollers. Because it pre-compiles the source and does not have an actual file system on the device for the files it uses a unique module resolution system that can be tricky sometimes to work with. In this case, because mod.ts was being imported in a secondary file, and mod.ts is the main entry point, it is unable to resolve the import. This PR resolves that issue.