This change makes SQLite queries (`open foo.db`, `open foo.db | query db
"select ..."`) cancellable using `ctrl+c`. Previously they were not
cancellable, which made it unpleasant to accidentally open a very large
database or run an unexpectedly slow query!
UX-wise there's not too much to show:
## Notes
I was hoping to make SQLite queries streamable as part of this work, but
I ran into 2 problems:
1. `rusqlite` lifetimes are nightmarishly complex and they make it hard
to create a `ListStream` iterator
2. The functions on Nu's `CustomValue` trait return `Value` not
`PipelineData` and so `CustomValue` implementations can't stream data
AFAICT.
57ff668d2e
