Return FunSpec.Builder from jvmModifiers to not break chaining

[Egorand]

Fixes #638.

• [x] docs/changelog.md has been updated if applicable.
  • Changes not visible to library consumers, such as build script, documentation, or test code updates, don't need to be added to the changelog.

[Egorand]

The plan is to ship this as part of 2.0, or 3.0, or do you think we shouldn't be breaking binary compat regardless? I'd actually like to clean up some deprecated APIs in either 2.0 or 3.0.

[JakeWharton]

I definitely would not break anything for 2.0. The sole behavior change is the wrapping change, though that doesn't break anything. In 3.0 we could move deprecations to be hidden. I don't want someone to have a reason not to upgrade due to compatibility.