Use stable sorting function

[anuramat]

Why not use stable sort for struct fields?

• Less comments would get misplaced
• The semantic ordering of fields would be often preserved:

  // something like
  type before struct {
  topBool    bool
  string1    string
  string2    string
  string3    string
  bottomBool bool
  }
  // sometimes turns into
  type after_bad struct {
  string2    string
  string1    string
  string3    string
  topBool    bool
  bottomBool bool
  }
  // when we can have
  type after_good struct {
  string1    string
  string2    string
  string3    string
  topBool    bool
  bottomBool bool
  }

The fix is like five keystrokes, but I'll attach a PR.

Thanks for your work btw, great tool!