rio/cmd/rio-ls-records: handle .rio.meta records when present
rio/cmd/rio-ls-records: handle .rio.meta records when present

This CL adds special handling of the '.rio.meta' record (which holds
records+blocks descriptions and their offsets).
When present, the rio metadata record is loaded and then used to display
the types of the other records:

```
$> rio-ls-records rivet.rio
rio-ls-records: inspecting file [rivet.rio]...
 -> EtaSumEt            type=*go-hep.org/x/hep/hbook.P1D
 -> Mult                type=*go-hep.org/x/hep/hbook.H1D
 -> Pt                  type=*go-hep.org/x/hep/hbook.H1D
 -> E                   type=*go-hep.org/x/hep/hbook.H1D
 -> Eta                 type=*go-hep.org/x/hep/hbook.H1D
 -> Rapidity            type=*go-hep.org/x/hep/hbook.H1D
 -> Phi                 type=*go-hep.org/x/hep/hbook.H1D
 -> MultCh              type=*go-hep.org/x/hep/hbook.H1D
 -> PtCh                type=*go-hep.org/x/hep/hbook.H1D
 -> ECh                 type=*go-hep.org/x/hep/hbook.H1D
 -> EtaCh               type=*go-hep.org/x/hep/hbook.H1D
 -> RapidityCh          type=*go-hep.org/x/hep/hbook.H1D
 -> PhiCh               type=*go-hep.org/x/hep/hbook.H1D
 -> EtaPMRatio          type=*go-hep.org/x/hep/hbook.S2D
 -> RapidityPMRatio     type=*go-hep.org/x/hep/hbook.S2D
 -> EtaChPMRatio        type=*go-hep.org/x/hep/hbook.S2D
 -> RapidityChPMRatio   type=*go-hep.org/x/hep/hbook.S2D
rio-ls-records: inspecting file [rivet.rio]... [done]
```

go-hep/hep,go-hep/hep