When creating/inferring schema only, do not buffer stdin

[corneliusroemer]

It's safest to infer the schema on the entire dataset.

When the dataset is larger than RAM, this is currently not possible via stdin as the implementation in #10 and #13 stores everything that's used for inference in memory.

In practice, one could stream the dataset via stdin twice: first time to get the schema, second time to convert.

This needs some internal changes to not buffer when options are set to infer schema only.