Fix all linter errors

- Add explicit error ignoring with _ = for Close/Remove calls
- Rename WriteTo to Write to avoid io.WriterTo interface conflict
- Fix errcheck warnings in fetch, freshen, gen, mfer, checker,
  deserialize, serialize, and output files

mfer/serialize.go

@@ -65,7 +65,7 @@ func (m *manifest) generateOuter() error {
 		return err
 	}
 
-	gzw.Close()
+	_ = gzw.Close()
 
 	o := &MFFileOuter{
 		InnerMessage:    idc.Bytes(),