Serialize and deserialize structs safely

There are a few places where we unnecessarily use unsafe blocks to serialize and deserialize structs:

In particular see https://github.com/oreboot/oreboot/blob/main/src/lib/uefi/src/lib.rs. For example:

let fveh: efi::EFI_FIRMWARE_VOLUME_EXT_HEADER =
                unsafe { core::ptr::read(fveh_bytes.as_ptr() as *const _) };

Instead, we should be using some Rust macros to achieve this safely. There are a number of crates built for this task:

• https://serde.rs/ is obviously the most popular. Some of its flexibility gets in the way. We just need to be able to serialize/deserialize a struct in the same format as a packed C struct.
• https://github.com/xoac/endian_codec
• https://crates.io/crates/packed_struct
• ... (I have no strong opinions for a particular crate at the moment)

Start in src/lib/uefi/src/lib.rs, especially the EFI_FIRMWARE_VOLUME_EXT_HEADER conversion shown in the issue, then locate the other unsafe struct serialization and deserialization sites. Compare the listed Rust crates for packed C-struct compatibility and determine the required byte layout and endianness. Done means the applicable conversions no longer use unsafe pointer reads while preserving the existing struct format.