Package-level declarations

Receiver of a cross-tick iterator { ... } snippet — scope and iterator in one object, as the stdlib's SequenceBuilderIterator is. Replaces the stdlib builder for two reasons:

Implicit receiver for every execute_code snippet: a shadowing println / print writing into the per-eval sink returned with the result. Kotlin resolves implicit-receiver members ahead of default-imported kotlin.io functions, so bare println(...) is captured for every argument type — while fully-qualified kotlin.io.println / System.out.println bypass the shadow onto the JVM's real stdout, and out of this eval's result.