blobby: replace iterators with const fn parsing

[newpavlov]

The new approach parses blb files into statics which allows to use the crate in no_std contexts. This PR additionally introduces the parse_into_structs macro which defines a struct with byte slice fields and transforms blobby data into list of these structs.

For example, it allows us to write:

blobby::parse_into_structs!(
    include_bytes!("/path/to/file.blb");
    static TEST_VECTORS;
    struct TestVector { key, nonce, aad, plaintext, ciphertext }
);

// transforms into:
struct TestVector {
    key: &'static [u8],
    nonce: &'static [u8],
    aad: &'static [u8],
    plaintext: &'static [u8],
    ciphertext: &'static [u8]
}

static TEST_VECTORS: &[TestVector] = { ... };

I also thought about adding support for byte array fields, but with the current Rust capabilities (i.e. without const traits) the resulting code becomes annoyingly complex, so I decided against it in the current version.

[newpavlov]

Unfortunately, with the current blb format to get number of items stored in a blb file we have to fully parse it. In other words, parse_into_slice effectively parses the file twice, which may be noticeably slow on large enough files. We could resolve this by adding number of stored items to the start of the file, but it would mean that we have to re-generate our existing blb files.