Rust BPF programs do not interface with the runtime via FFI-safe types

[jackcmay]

Problem

When calling invoke the program passes AccountInfo structures to the runtime. AccountInfo is not-FFI safe and is not ABI stable.

Some context: #14523

Proposed Solution

All types going in and out of the program should be FFI-safe and ABI stable. In short, they should use Repr(C) representation.

• Input parameters are currently serialized into a data buffer and therefore safe but there is an effort to rework this interface to reduce copies (Program serialization is inefficient due to copies of accounts into and out of the program #14691). Depending on what that rework looks like any types passed to the entrypoint should be safe
• The only output parameter is a u64 which safe
• Parameters passed to Invoke are pointers to internal structures which are then translated and recreated by the runtime. Since AccountInfo is Repr(C) this translation is not stable. All types passed into and out of invoke must be safe.