Rework RE2::Arg with templates instead of macros.

Bump SONAME accordingly.

Change-Id: Ic62e8de87dda68951c2a49715a826c8f42c2dbe3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57850
Reviewed-by: Paul Wankadia <junyer@google.com>
4 files changed