Filter out keys when processing objects

[lemire]

The idea is to very quickly dismiss keys that are not a match when searching through an object.

Suppose that the target has at least 7 bytes.

Pick up a 64-bit word. Copy up to 8 characters from the target to the this 64-bit word, terminating with a quote if possible. Let us call this word FILTER.

Go through the keys. Load a 64-bit word (always safe because of padding) at that location. Do an XOR with FILTER. You get zero if and only if the first 8 characters match (including possible the quote). If so, investigate further, if not move on.

If the target has fewer than 7 bytes, then you need to use a mask, but there are only 7 cases so you can use a lookup table or some other cleverness.

See #1485

[lemire]

@jkeiser Marking for 1.0 though it could be 0.9.

It is not hard to implement. It just requires benchmarking.

[lemire]

Note that this could be extended to handle escaped content (backslashes), maybe optionally.

[jkeiser]

It seems like this might also play nicely with multi-field lookup interface ("find field x, y or z, whatever comes next").

[lemire]

@jkeiser Right. So I imagine a template where you pass a "matcher" which returns true if and only if the string matches. The idea is to do more work upfront building a matching instance. Ultimately, you could do regex!!!