We use ArrayKind as a simple unsigned integer, its an index, we do
range-based comparisons, and in the future we'll likely do bit-ops
for fast sub-kind tests. Just be honest that this is an old-school enum.
Introduces a new m_kind for vector-shaped HphpArray instances.
A vector has integer indexes in the range [0..size), and no
tombstones. Internally it does not store the integer keys or
hashtable, although this version still allocates space for it.
We'd like to start using ##mixed## instead of ##var## for attribute types to be consistent with Hack. As a followup to this (once released), we would codemod all ##var## to ##mixed##.
Sean Cannella