cmd/compile: add setNodeSeq, and use it in bimport.go

cmd/compile: add setNodeSeq, and use it in bimport.go

Also rewrite bexport.go to use nodeSeqIterate.

The new setNodeSeq is a transitional generic function to set either a
NodeList or a slice to either a NodeList or a slice.  This should permit
us to flip fields from *NodeList to []*Node, or Nodes, without changing
other code.

Passes toolstash -cmp.

Change-Id: I872cbfe45bc5f432595737c1f6da641c502b1ab6
Reviewed-on: https://go-review.googlesource.com/20194
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>