Refactor all unsafe collection enumerators to use ref struct types,
removing support for boxing and standard .NET enumeration interfaces.
GetEnumerator methods now return stack-only, more efficient enumerators
with [UnscopedRef] and inlining attributes. IEnumerable<T> and
IEnumerable implementations are removed from affected types. Interfaces
now require unmanaged types. Also includes minor doc and bug fixes.
BREAKING CHANGE: Enumerators are no longer compatible with LINQ, and collections no longer implement IEnumerable/IEnumerator.
