[vm/ffi] add alignmentOf<T> (besides just having sizeOf<T>)

[dcharkes]

Allocation in package:ffi currently only relies on the sizeOf an FFI type. However, when allocating more than count: 1, it should also take into account the required alignment.

For example, the size of of a struct might be 28 bytes, while the required alignment should be 8 bytes. This means that when allocating count: 2, the second value should be at an offset of 32 bytes, rather than 28 bytes.

Proposed Solution

Add the alignmentOf<T>:

/// Number of bytes used by native type T.
///
/// Includes padding and alignment of struct fields.
external int sizeOf<T extends NativeType>();

/// Alignment in number of bytes of native type T.
///
/// Includes padding and alignment of structs.
external int alignmentOf<T extends NativeType>();

Note that we might want to remove the generic use of sizeOf<T> (#38721 (comment)), the same would be true for alignmentOfT<T>.

Original Example

class KeyboardInput extends Struct {
  @Uint32() int type;
  @Uint16() int virtualCode;
  @Uint16() int scanCode;
  @Uint32() int flags;
  @Uint32() int time;
  Pointer dwExtraInfo; // Is 8 bytes long and 8 byte aligned on many 64 bit platforms.
  @Uint32() int padding;
}

I don't understand why sizeOf<KeyboardInput>() gives me size of 32 instead of 28 (but with padding field removed it correctly says 24).

Originally posted by @marad in #37271 (comment)

Thanks for reporting this @marad.

cc @mkustermann

[dcharkes]

We should probably do this together with #36730, they both need the same info.

[safasofuoglu]

@dcharkes is it possible this gets attention anytime soon?