Closes#7581.
After this PR, `describe` shows `(stream)` next to input that arrived at
`describe` as a `ListStream`:
```bash
〉ls | describe
table<name: string, type: string, size: filesize, modified: date> (stream)
〉[1 2 3] | each {|i| $i} | describe
list<int> (stream)
```
`describe` must collect all items of the stream to display type
information for lists and tables. If users need to avoid collecting
input, they can use the `-n`/`--no-collect` flag:
```bash
〉[1 2 3] | each {|i| $i} | describe --no-collect
stream
```
95d4922e44