Do a large chunk of the move to go

This commit moves a huge chunk of the code to go. The TODO.md document
indicates what is left to be done.

cmd/shell.go

@@ -0,0 +1,49 @@
+package cmd
+
+import (
+	"fmt"
+
+	wisski_distillery "github.com/FAU-CDI/wisski-distillery"
+	"github.com/FAU-CDI/wisski-distillery/env"
+	"github.com/tkw1536/goprogram/exit"
+	"github.com/tkw1536/goprogram/parser"
+)
+
+// Shell is the 'shell' command
+var Shell wisski_distillery.Command = shell{}
+
+type shell struct {
+	Positionals struct {
+		Slug string   `positional-arg-name:"SLUG" required:"1-1" description:"slug of instance to show run shell in"`
+		Args []string `positional-arg-name:"ARGS" description:"arguments to pass to the shell"`
+	} `positional-args:"true"`
+}
+
+func (shell) Description() wisski_distillery.Description {
+	return wisski_distillery.Description{
+		Requirements: env.Requirements{
+			NeedsConfig: true,
+		},
+		ParserConfig: parser.Config{
+			IncludeUnknown: true,
+		},
+		Command:     "shell",
+		Description: "Open a shell in the provided instance",
+	}
+}
+
+func (sh shell) Run(context wisski_distillery.Context) error {
+	instance, err := context.Environment.Instance(sh.Positionals.Slug)
+	if err != nil {
+		return err
+	}
+
+	code := instance.Shell(context.IOStream, sh.Positionals.Args...)
+	if code != 0 {
+		return exit.Error{
+			ExitCode: exit.ExitCode(uint8(code)),
+			Message:  fmt.Sprintf("Exit code %d", code),
+		}
+	}
+	return nil
+}