exp: no check for vanity import

[sbinet]

it seems gonum.org/v1/exp/... packages are lacking vanity import decoration.
e.g.:

• https://github.com/gonum/exp/blob/master/plotter/rings/rings.go#L10
• https://github.com/gonum/exp/blob/master/dframe/dframe.go#L13
• https://github.com/gonum/exp/blob/master/linsolve/doc.go#L18