Optimise the layout of the `RE2` class.

In particular, `error_arg_` should be a pointer... that will almost
always be `empty_string`. Also, separate the relatively cold fields
from the relatively hot fields to be slightly friendlier to caches.

Bump SONAME accordingly.

Credit and gratitude to Clement Courbet for his suggestions.

Change-Id: I6553878bbae5e2aae650ad37a7755a833708af4b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60190
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
4 files changed