#[repr(C)] structs missing Cread/Cwrite implementations

All the `#[repr(C)]` structs implement Pread/Pwrite but do not implement Cread/Cwrite which is useful in no_std environments.

I'd be happy to implement Cread/Cwrite for the ELF structs (with tests), but wanted to know if this is something that is wanted for the project and if the implementation below is the recommend way to do this.

Here is my implementation for the ELF header:
```rust
use core::mem;
use scroll::{Cread, Cwrite, Endian};

impl ctx::FromCtx for Header {
#[inline]
fn from_ctx(src: &[u8], le: Endian) -> Self {
assert!(src.len() >= SIZEOF_EHDR);
let mut elf_header = Header::default();
for i in 0..SIZEOF_IDENT {
elf_header.e_ident[i] = src.cread(i);
}
let endianness = match elf_header.e_ident[EI_DATA] {
ELFDATA2LSB => scroll::LE,
ELFDATA2MSB => scroll::BE,
// fallback to provided endian
_ => le,
};
elf_header.e_type = src.cread_with(SIZEOF_IDENT, endianness);
elf_header.e_machine = src.cread_with(SIZEOF_IDENT + 2, endianness);
elf_header.e_version = src.cread_with(SIZEOF_IDENT + 4, endianness);
elf_header.e_entry = src.cread_with(SIZEOF_IDENT + 8, endianness);
elf_header.e_phoff =
src.cread_with(SIZEOF_IDENT + 8 + mem::size_of::<$size>(), endianness);
elf_header.e_shoff =
src.cread_with(SIZEOF_IDENT + 8 + 2 * mem::size_of::<$size>(), endianness);
...
elf_header.e_shstrndx =
src.cread_with(SIZEOF_IDENT + 22 + 3 * mem::size_of::<$size>(), endianness);
elf_header
}
}
impl ctx::IntoCtx for Header {
fn into_ctx(self, bytes: &mut [u8], le: Endian) {
assert!(bytes.len() >= SIZEOF_EHDR);
let endianness = match self.e_ident[EI_DATA] {
ELFDATA2LSB => scroll::LE,
ELFDATA2MSB => scroll::BE,
// fallback to provided endian
_ => le,
};
for i in 0..self.e_ident.len() {
bytes.cwrite(self.e_ident[i], i);
}
bytes.cwrite_with(self.e_type, SIZEOF_IDENT, endianness);
bytes.cwrite_with(self.e_machine, SIZEOF_IDENT + 2, endianness);
...
bytes.cwrite_with(
self.e_shstrndx,
SIZEOF_IDENT + 22 + 3 * mem::size_of::<$size>(),
endianness,
);
}
}
```

Start by locating the ELF structs marked #[repr(C)] and their existing Pread/Pwrite implementations. Review the proposed Header Cread/Cwrite approach and the project's current tests before extending it to the relevant structs. Done means the intended ELF structs support Cread/Cwrite and have coverage for the new behavior.

Written by the indexing model from the issue text.