internal/impl: rename encode->codec in various places

The code organization is simpler if we keep the functions encoding and
decoding a particular type (e.g., maps) together rather than split
across files. Rename various "encode" files to "codec" in preparation
for adding fast-path decoding.

Change-Id: If1e271da99d31533ffefc19b1fc847936fa9484a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185241
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/cmd/generate-types/impl.go

@@ -8,11 +8,11 @@ import (
 	"text/template"
 )
 
-func generateImplEncode() string {
-	return mustExecute(implEncodeTemplate, ProtoKinds)
+func generateImplCodec() string {
+	return mustExecute(implCodecTemplate, ProtoKinds)
 }
 
-var implEncodeTemplate = template.Must(template.New("").Parse(`
+var implCodecTemplate = template.Must(template.New("").Parse(`
 {{- /*
   IsZero is an expression testing if 'v' is the zero value.
 */ -}}

internal/cmd/generate-types/main.go

@@ -38,7 +38,7 @@ func main() {
 
 	chdirRoot()
 	writeSource("internal/filedesc/desc_list_gen.go", generateDescListTypes())
-	writeSource("internal/impl/encode_gen.go", generateImplEncode())
+	writeSource("internal/impl/codec_gen.go", generateImplCodec())
 	writeSource("proto/decode_gen.go", generateProtoDecode())
 	writeSource("proto/encode_gen.go", generateProtoEncode())
 	writeSource("proto/size_gen.go", generateProtoSize())