Wasteful Vec allocation in read_string and read_vec

[dtolnay]

Every String and Vec<u8> deserialization (as of #115) allocates a fresh vector. From what we've seen in the JSON deserializer, reusing a long-lived buffer would be a huge improvement.

[khuey]

This optimization would only affect the deserialize_bytes case. Both deserialize_byte_buf and deserialize_string want to consume the buffer to produce zero-copy output.

[TyOverby]

@khuey: also deserialize_str

[khuey]

Yes, forgot that one.