Streaming/Lazy/No-copy scanner.

I tried FallibleStreamingIterator but failed due to some errors reported by the borrow checker. But our Scanner is a FallibleStreamingIterator:

FallibleStreamingIterator differs from the standard library's Iterator trait in two ways: iteration can fail, resulting in an error, and only one element of the iteration is available at any time. While these iterators cannot be used with Rust for loops, while let loops offer a similar level of ergonomics.

Currently, there are one unsafe block in the scan method used to bypass the borrow checker. I don't know if it can be replaced with safe code. But I am quite confident that it is safe.

Two concrete scanners are implemented:

• CSV lexer based on SQLite extension (use rust-csv instead),
• SQL lexer based on SQLite tokenizer.