script: Use strings instead of byte slice literals in asset generation

It compiles literally 50 times faster and generates the same code. Also
add the little header that indicates auto generated code.

script/genassets.go

@@ -23,7 +23,9 @@ import (
 	"time"
 )
 
-var tpl = template.Must(template.New("assets").Parse(`package auto
+var tpl = template.Must(template.New("assets").Parse(`// Code generated by genassets.go - DO NOT EDIT.
+
+package auto
 
 const Generated int64 = {{.Generated}}
 
@@ -70,7 +72,7 @@ func walkerFor(basePath string) filepath.WalkFunc {
 			name, _ = filepath.Rel(basePath, name)
 			assets = append(assets, asset{
 				Name: filepath.ToSlash(name),
-				Data: fmt.Sprintf("%#v", buf.Bytes()), // "[]byte{0x00, 0x01, ...}"
+				Data: fmt.Sprintf("[]byte(%q)", buf.String()),
 			})
 		}