Driver scanning API

[ncruces]

golang/go#67546 (comment)

[ncruces]

If this gets merged upstream, add:

• bool if it causes allocs (it probably doesn't)
• float32
• NullFoo for the foo
• Null[foo] for the foo (including int)

It's probably not worth supporting unsigned ints, or 32 and 16-bit ints; 8-bit ints should avoid allocs already.

Their use is less common, and I'd rather avoid replicating the overflow behavior (which is why int is only supported on 64-bit platforms).

I could revaluate unsigned. But the error path would go through the fallback.

NullFoo I want to support because it avoids allocs, in a way *foo never will.