FFI API Audit

[gatesn]

• #[deny(missing_docs)] in vortex-ffi/lib.rs
• Define vx_ptype_t enum for variant
• Define vx_dtype_t enum for variant.
• Typedef vx_nullable_t to true/false enum.
• vx_dtype_new should actually be vx_dtype_new_primitive(vx_ptype_t, nullable_t).
• vx_dtype_get shoule be vx_dtype_t vx_dtype_variant(const vx_dtype *dtype)
• Add vx_string type, without null-termination. Use this for all strings. Add a macro to construct vx_string from compile-time constant C str
• Timestamp types, blergh. See Epic: Extension DTypes, Scalars, and Arrays #2989
• Replace ArrayStream with ArrayIterator since the API is blocking. Use ScanBuilder::into_array_iter
• Maybe hide file open / scan APIs inside DuckDB feature flag. As in, they're not public API yet since they're not stabilised in Rust even.
• Use usize_t instead of uint64_t where usize is used in Rust. Else WASM API gets weird. We also use uint32 in a bunch of places that should also be usize_t, e.g. vx_array_null_count
• Improve docs with consistent way of describing ownership.
• Seems like we use struct <type_name> everywhere instead of the typedefs?
• Perhaps rename vx_array_free and similar functions to _drop, since we often Arc internally it's not true that this call will always trigger a free. Don't mind either way. For Arc we should probably also add vx_T_clone too though.
• Options structs should be opaque, with setter function that takes an option enum? Or individual setters? Not sure.

I still don't think we have error handling quite right. When propagating errors, I want to be able to write something like:

vx_result my_function(..., vx_err *err_out) {
  ...
  if !vx_something_blah(..., err_out) {
    return VXErr;
  }
}