Align StreamDeserializer with serde_json

Would be nice to have the exact same API for StreamDeserializer as serde_json, currently missing these two methods:
```rust
/// Create a JSON stream deserializer from one of the possible serde_json input sources.
pub fn new(read: R) -> Self
/// Returns the number of bytes so far deserialized into a successful T.
pub fn byte_offset(&self) -> usize
```