Most could use the each method, but because of the hack used to
disambiguate old- and new-style loops, some had to use vec::each.
(This hack will go away soon.)
Issue #1619
1. Enforce mutability declarations on class fields. Don't allow any
mutation of class fields not declared as mutable (except inside the
constructor).
2. Handle classes correctly in shape (treat classes like records).
Cross-crate metadata for classes works well enough that programs with
classes in other crates compile successfully, but output wrong results.
Checking in work so far to avoid merge hassles. (Tests are xfailed.)
