TODO remove this once there's a standard jenny for this...somewhere in core.

NewTargetJennies initializes a new TargetJennies with appropriate namers for each JennyList.

Prefixer returns a FileMapper that injects the provided path prefix to files passed through it.

SlashHeaderMapper produces a FileMapper that injects a comment header onto a [codejen.File] indicating the main generator that produced it (via the provided maingen, which should be a path) and the jenny or jennies that constructed the file.

TargetJennies is a set of jennies for a particular target language or tool that perform all necessary code generation steps.