Update vg rna to project reasonably onto haplotypes containing CNVs

[jeizenga]

Pertains to #4397.

To do this, we need to be able to identify predecessor/successor on the output of locate queries from the GBWT to enforce colinearity constraints on the haplotype sequences through exons. @jltsiren tells me that this is possible with the R-index. However, this will require building the R-index alongside the GBWT in the vg rna pipeline, as well as its implementation in vg autoindex.