Noteworthy features of Starlark

Starklark is Bazel's configuration language, and not designed to be general-purpose. Nontheless, there are some features that seem useful even for a dynamic G/P language.

1. Single final assignment at the top level. Module-global functions, variables cannot be re-bound. This helps reading code, and simplifies tooling.
2. Deterministic iteration order for dictionaries, and in general determinism (a program run twice always produces the same outcome, modulo things like time). Determinism seems like a generally desirable property, for things like testing/reproducible builds.
3. No mutation of iterator during iteration. Mutating the iterator (like a list) during iteration panics the program, to avoid iterator invalidation errors.
4. No [checked] exceptions. Panicking the program for any non-anticipated error might seem problematic, but "makes the language simpler and reduces the number of concepts." Exceptions also become API surface for the language, so not having it helps language evolution.
5. Strings are not iterable. This avoids bugs from passing in a string instead of a length-1 list of strings to APIs expecting a list.