Improve Kotlin CASE DSL

[jeffgbutler]

This PR changes the "else" and "then" functions to infix. In addition, we simplify "then" functions in the simple CASE expression.

Previously...

select(case(id) {
    `when`(1, 2, 3) { then(true) }
    `else`(false)
  } `as` "small_id"
) {
    from(foo)
}

Now...

select(case(id) {
    `when`(1, 2, 3) then true
    `else`(false)
  } `as` "small_id"
) {
    from(foo)
}

[coveralls]

coverage: 100.0%. remained the same
when pulling 15d258e on jeffgbutler:improve-kotlin-case-dsl
into ecf4bc3 on mybatis:master.