gpython: leverage runtime/debug.ReadBuildInfo to infer version

starting with Go-1.12, there's a nifty function [runtime/debug.ReadBuildInfo](https://golang.org/pkg/runtime/debug/#ReadBuildInfo) to extract build informations out of a Go binary.

we could leverage this to display:
```
$> gpython
Python 3.4.0 (none, unknown)
[Gpython (devel)]
- os/arch: linux/amd64
- go version: devel +4f13a9c5b1 Tue Oct 1 07:16:47 2019 +0000
>>>
```

(notice the `(devel)` string that comes out of `debug.ReadBuildInfo().Main.Version`

this could be factored into a `gpython/vm.Version()` function:
```go
// Copyright 2019 The go-python Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build go1.12

package vm

import (
"fmt"
"runtime/debug"
)

// Version returns the version of Gpython and its checksum. The returned
// values are only valid in binaries built with module support.
//
// If a replace directive exists in the Gpython go.mod, the replace will
// be reported in the version in the following format:
// "version=>[replace-path] [replace-version]"
// and the replace sum will be returned in place of the original sum.
//
// The exact version format returned by Version may change in future.
func Version() (version, sum string) {
b, ok := debug.ReadBuildInfo()
if !ok {
return "", ""
}

const root = "github.com/go-python/gpython"

modules := append([]*debug.Module{&b.Main}, b.Deps...)

for _, m := range modules {
if m.Path == root {
if m.Replace != nil {
switch {
case m.Replace.Version != "" && m.Replace.Path != "":
return fmt.Sprintf("%s=>%s %s", m.Version, m.Replace.Path, m.Replace.Version), m.Replace.Sum
case m.Replace.Version != "":
return fmt.Sprintf("%s=>%s", m.Version, m.Replace.Version), m.Replace.Sum
case m.Replace.Path != "":
return fmt.Sprintf("%s=>%s", m.Version, m.Replace.Path), m.Replace.Sum
default:
return m.Version + "*", m.Sum + "*"
}
}
return m.Version, m.Sum
}
}

return "", ""
}
```