use right value instead of copy value in c++ interface

[stdpain]

use move instead of copy in c++ interface which will get good performance under sparse bitmap
StarRocks/starrocks#86

[lemire]

I like it a lot, but I'd like an independent review before merging.

@Oppen would you have a look? It short.

[Oppen]

It seems insert also implements r-value reference versions, so if insertOrEmplace is only for internal use, it may be better to just add an r-value reference version and always use std::move in that function. Does that make sense?
I can probably be done that way regardless of it being internal or not, at least Godbolt won't err on a dummy case.

[stdpain]

@Oppen Applied

[Oppen]

LGTM!

[lemire]

This is nice. Merging.