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

internal/checker/checker.go

@@ -218,7 +218,7 @@ func (c *Checker) checkFile(entry *mfer.MFFilePath, checkedBytes *int64) Result
 	if err != nil {
 		return Result{Path: entry.Path, Status: StatusError, Message: err.Error()}
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	h := sha256.New()
 	n, err := io.Copy(h, f)