encode: Encode nested tuples properly

[lexnv]

This PR ensures that nested tuples inside sequences can be encoded properly.

Prior to this encoding a Vec<(u8, u16)> would not work properly with dynamic values of

Value::unnamed_composite(
        vec![
            Value::u128(3u8.into()),
            Value::u128(1u16.into()),
        ]
    );

This is because find_sequence_candidate function did assume that the last type of the value is the "end-type" of the sequence.

Instead, if we deal with tuple/composite inside a sequence (sequence or array otherwise), then recurse back to encode_composite with proper parameters (unflattened).

Detected by: paritytech/subxt#982.

// @paritytech/subxt-team

[niklasad1]

looks good to me but obviously not super familiar with the code

[jsdw]

I propose closing this in favour of #32; I had a look for a while and wanted to come up with an approach to simplify the method we use to encode things slightly since it was already quite complicated and had a bunch of other edge cases where it would fail to encode things properly :)

[lexnv]

Yep, closing this in favor of #32 👍