ci: add go1.18 go version

graph/multi: use more efficient and correct direction dedup logic

Also add test case to testgraph corpus.

spatial/vptree: don't do no work when n is greater than len(s)

graph/flow: improve consistency of coverage reports

modules,graph/layout: update for gonum/plot v0.9.0

graph/encoding/dot: factor out repeated forward keys

lapack,lapack64: remove Dlansb and Dlantb from Float64

These two routines are not provided by LAPACKE and therefore
should not be part of the Float64 interface.

Closes gonum#1428

ci: bump travis, appveyor and module versions

Also remove redundant Go version build tag.