Function zcash_deserialize_bytes_external_count 

pub fn zcash_deserialize_bytes_external_count<R: Read>(
    external_count: usize,
    reader: R,
) -> Result<Vec<u8>, SerializationError>

zcash_deserialize_external_count, specialised for raw bytes.

This allows us to optimize the inner loop into a single call to read_exact().

This function has a zcash_ prefix to alert the reader that the serialization in use is consensus-critical serialization, rather than some other kind of serialization.