Streaming tables

Up to this point, we've taken a few shortcuts with how streams worked in Nushell. We knew that a table, in theory, was a stream of rows. In practice, though, this wasn't the case as the code has lots of assumptions about how tables worked.

With the 0.4.0, we're taking a big step in the direction of full support for streams. You'll see that now data will start being printed as it becomes available, with tables printing pages of 50 rows at a time (by default). This allows you to get deep listings of files (eg using `ls **`), work with streams from external sources, and more.