build(gnovm): improve code generation (#3595)

- follow the standard `^// Code generated .* DO NOT EDIT\.$` syntax in
genstd (see `go help generate`)
- move `stringer` to `go:generate` directive, output all String methods
into one single file

gnovm/Makefile

@@ -125,27 +125,15 @@ _test.filetest:;
 
 ########################################
 # Code gen
-# TODO: move _dev.stringer to go:generate instructions, simplify generate
-#       to just go generate.
 .PHONY: generate
-generate: _dev.stringer _dev.generate _dev.docs fmt imports
+generate: _dev.generate _dev.docs fmt imports
 
 .PHONY: _dev.docs
 _dev.docs:
 	mkdir -p .tmp
 	(go run ./cmd/gno -h 2>&1 || true) | grep -v "exit status 1" > .tmp/gno-help.txt
 	$(rundep) github.com/campoy/embedmd -w `find . -name "*.md"`
 
-stringer_cmd=$(rundep) golang.org/x/tools/cmd/stringer
-.PHONY: _dev.stringer
-_dev.stringer:
-	$(stringer_cmd) -type=Kind       ./pkg/gnolang
-	$(stringer_cmd) -type=Op         ./pkg/gnolang
-	$(stringer_cmd) -type=TransCtrl  ./pkg/gnolang
-	$(stringer_cmd) -type=TransField ./pkg/gnolang
-	$(stringer_cmd) -type=VPType     ./pkg/gnolang
-	$(stringer_cmd) -type=Word       ./pkg/gnolang
-
 .PHONY: _dev.generate
 _dev.generate:
 	go generate -x ./...

gnovm/pkg/gnolang/doc.go

@@ -1,2 +1,7 @@
 // SPDX-License-Identifier: GNO License Version 1.0
+
+// Package gnolang contains the implementation of the Gno Virtual Machine.
 package gnolang
+
+//go:generate -command stringer go run -modfile ../../../misc/devdeps/go.mod golang.org/x/tools/cmd/stringer
+//go:generate stringer -type=Kind,Op,TransCtrl,TransField,VPType,Word -output string_methods.go .

gnovm/pkg/gnolang/machine.go

@@ -1,7 +1,5 @@
 package gnolang
 
-// XXX rename file to machine.go.
-
 import (
 	"fmt"
 	"io"