Rename Array VTables

[gatesn]

We are about to move more functions onto the VTable struct. To make this API nicer for users we're renaming FooVTable to Foo. Since users don't really interact with these structs at the moment, it should have a relatively small blast radius.

That said, anyone who has code like this should update it

- if let Some(foo) = array.as_opt::<FooVTable>() { ... }
+ if let Some(foo) = array.as_opt::<Foo>() { ... }