mkfs serialize/de-serialize manifest support

[eyberg]

the new go native mkfs lib deprecates the old text-based serialization format we used in generating manifests for mkfs to consume (for ops)

it's still useful for debugging purposes though to hand-edit manifests from time to time for larger projects where ops can print out what was generated and then hand edit it - c based mkfs still works but is always passed hand-edited manifests

this is issue to track the re-addition of this capability for ops