Improved in memory streaming encryptor

[grumbach]

    let mut stream = stream_encrypt(data_size, data_iter)?;

    // Collect all chunks
    for chunk_result in stream.chunks() {
        let (xorname, chunk) = chunk_result?;
        // process chunks
    }

    // Get the datamap
    let datamap = stream.datamap()?;

[maqi]

Though this PR is on top of the PR 421, the capability of out-of-order content push got removed (and the correspondent test as well.)

The stream_encrypt_from_file shall utilised the newly StreamEncrypt (or EncryptionStream) struct as well (like the PR 421 does).
So that we can avoid having too many different workflow path, which results in headache for future maintaince, like what we had in ant_cli/autonomi_client .

Regarding the stream_encrypt , I'd suggest to make it as a one-off help function as well (similar to stream_encrypt_from_file ):

fn stream_encrypt(data_size: usize, bytes_iterator: I, chunk_store: S) -> Result<DataMap>;

which we then can use it like this:

let datamap = stream_encrypt.datamap(data_size, bytes_iterator, chunk_store)?;

[maqi]

As discussed in the HO,
it is decided that:

• out-of-order content push is not mandatory for streaming_encrypt
• simpler API is preferred, unless there is critial user requirement to be asked for

Hence, got this PR approved to replace the PR 421