Add streaming API which works with the basic and compat APIs about simdutf8 HOT 2 OPEN

This is partially implemented in v0.1.3 as low-level API in simdutf8::basic::imp.

Still missing:

• compat API with early validation failure and exact error information.
• Safe API with implementation auto-selection.

from simdutf8.

Question: would it be possible to implement a transparent wrapper around BufReader with such an API? I'm thinking about using it for quick-xml, which I think it would be well suited for. Essentially:

• Raw bytes get progressively validated as UTF-8 as they stream out of the BufReader
• The XML parsing operates on raw bytes, searching for the standard characters such as < and >
• Knowing that the input is valid UTF-8, we can safely use std::str::from_utf8_unchecked() as needed so long as we use the known character boundaries from parsing

from simdutf8.