Provide a way to skip interface conversions

There's a bug in how they're generated which I don't want to fix right
now (and the interface in question is disk.Disk).

cmd/goose/main.go

@@ -86,6 +86,7 @@ func main() {
 	flag.BoolVar(&tr.AddSourceFileComments, "source-comments", false,
 		"add comments indicating Go source code location for each top-level declaration")
 	flag.BoolVar(&tr.TypeCheck, "typecheck", false, "add type-checking theorems")
+	flag.BoolVar(&tr.SkipInterfaces, "skip-interfaces", false, "skip creating interface conversions")
 
 	var outRootDir string
 	flag.StringVar(&outRootDir, "out", ".",

goose.go

@@ -2190,8 +2190,10 @@ func (ctx Ctx) maybeDecls(d ast.Decl) []coq.Decl {
 	switch d := d.(type) {
 	case *ast.FuncDecl:
 		var cvs []coq.Decl
-		for _, stmt := range d.Body.List {
-			cvs = ctx.stmtInterface(cvs, stmt)
+		if !ctx.SkipInterfaces {
+			for _, stmt := range d.Body.List {
+				cvs = ctx.stmtInterface(cvs, stmt)
+			}
 		}
 		fd := ctx.funcDecl(d)
 		var results []coq.Decl

interface.go

@@ -176,6 +176,7 @@ func sortedFiles(fileNames []string, fileAsts []*ast.File) []NamedFile {
 type TranslationConfig struct {
 	TypeCheck             bool
 	AddSourceFileComments bool
+	SkipInterfaces        bool
 }
 
 func pkgErrors(errors []packages.Error) error {