message: Add Globals::default() being the standard globals
  Summary: Useful for writing simpler introductory programs.
diff: use crate::{
  collections::SmallMap,
  + stdlib::standard_environment,
  values::{
  structs::FrozenStruct, AllocFrozenValue, FrozenHeap, FrozenHeapRef, FrozenValue, Value,
  },
  @@ -35,6 +36,12 @@ struct GlobalsData {
  variables: HashMap<String, FrozenValue>,
  }
  +impl Default for Globals {
  + fn default() -> Self {
  + standard_environment().build()
  + }
  +}
  +
  // Why are these things RefCell? Because we need to allocate things from the heap
  // which returns values pointing at the heap, and thus are an immutable borrow.
  // Once you have that, you can't mutably borrow at the same time, so have to use
  