Add support for cx_hkdf functions

[ndcroos]

Bindings for the hkdf methods defined here are currently missing in the Rust SDK.
I've written the following bindings. Does this seem good and should I submit this as a pull request (for example in a separate file named 'hkdf.rs')?

use nanos_sdk::bindings::*;

extern "C" {
    pub fn cx_hkdf_extract(hash_id: cx_md_t, ikm: &[u8], ikm_len: u32, 
        salt: &[u8], salt_len: u32, prk: &[u8]);

    pub fn cx_hkdf_expand(hash_id: cx_md_t, prk: &[u8], prk_len: u32, info: &[u8], 
        info_len: u32, okm: &[u8], okm_len: u32);
}

/// Wrapper for cx_hkdf_extract
/// 
/// Extract entropy 
/// 
/// In: 
/// ikm: input key material (acting as data)
/// salt: salt acting as a key
/// prk: pseudorandom key
pub fn hkdf_extract(ikm: &[u8], salt: &mut [u8], prk: &mut [u8]) {
    unsafe {
        cx_hkdf_extract(
            CX_SHA256,
            ikm.as_ptr(),
            ikm.len() as u32,
            salt.as_mut_ptr(),
            salt.len() as u32,
            prk.as_mut_ptr()
        );
    }
}

/// Wrapper for cx_hkdf_expand
/// 
/// Expand generated output of an already reasonably random input 
/// 
/// In: 
/// prk: pseudorandom key
/// info: 
/// okm: output key material
pub fn hkdf_expand(prk: &[u8], info: &[u8], okm: &mut [u8]) {
    unsafe {
        cx_hkdf_expand(
            CX_SHA256,
            prk.as_ptr(),
            prk.len() as u32,
            info.as_mut_ptr(),
            info.len() as u32,
            okm.as_mut_ptr(),
            okm.len() as u32
        );
    }
}

[yeti-g]

@ndcroos Nice! PR would be great.

[yhql]

Hi, thanks for providing code!

These bindings would be invalid however, as the extern C part should use only raw pointers, and use mut when necessary:
salt: &[u8] should be alt: *mut u8 for example.
(read-only should be *const u8)

This "extern "C"" prototype could be generated automatically by adding libcxng/src/cx_hkdf.c to the list of compiled sources in the build.rs script, and libcxng/src/cx_hkdf.h to the bindgen headers