`gene_locus_list`: add to echoverse

[bschilder]

This function got lost at some point, but it's used in the QTL vignette

#' Generate a named list of [e]gene-locus pairs
#'
#' @export
gene_locus_list <- function(top_SNPs){
  setNames(top_SNPs$Locus, top_SNPs$Gene)
}