Bug 14386 – add setOrGet to AA

The function is called setdefault in python and getOrElseUpdate in Scala. Allows to lookup a key or set a value with a single hash. This is also possible in C++ using iterators. https://docs.python.org/2/library/stdtypes.html#dict.setdefault http://www.scala-lang.org/api/2.10.1-RC1/scala/collection/mutable/HashMap.html#getOrElseUpdate(A,⇒B):B Another primitive that could work here is getLValue, but it requires the value type to have a distinct null value, so it's less nice to work with.

Since https://github.com/dlang/druntime/commit/0c92d13c7f8540bd91c3cce251d97ff39b84a486 (present in 2.082) there is now a require() function that does exactly that.