Also support "updater" ("modifier")?

[ymtszw]

The title says all.

In README I wrote:

• It does not provide "updaters" in the sense of (a -> a) -> { b | name : a } -> { b | name : a }
  • It might prove useful, though my current intuition says it sees less usages than setters.

But if the need arises often enough, I'm OK to include the feature. Do please rate this issue up to show the need!

• It should be opt-in. To keep the code-generation behavior backward-compatible.
• Opt-in should be done via command-line argument. Like --updater