[g]hobject_t: normalize operator<< result, and implement parse

[g]hobject_t: normalize operator<< result, and implement parse

- print hash in *bitwise reverse* order
- include all fields, every time
- use : delimitor for hobject_t
- use @ delimitor for ghobject_t
- escape unprintable characters <32, >=127, :, %.
  - also /, for fuse layer's benefit

Signed-off-by: Sage Weil <sage@redhat.com>