Upgrade golang.org/x/tools for compatibility with Go 1.14.

[atombender]

Upgrades golang.org/x/tools to v0.0.0-20200423205358-59e73619c742. Fixes cases such as this:

$ cat <<end > main.go
 package main

import (
    "errors"
)

func main() {
    _ = errors.New("x")
}
end
$ go-sumtype main.go
internal error: package "errors" without types was imported from "command-line-arguments"

[ghostsquad]

any movement on this?

[sudarshan-reddy]

@BurntSushi you are probably busy but it might be good to have this merged because using go-sumtype actively errors with newer versions of Go.