release 2021-02-01

Use `static_cast<>` instead of a C-style cast.

Fixes #287.

Change-Id: I8b67c0b5088bb1a06a47017528bfd8e6f1ec4cca
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58230
Reviewed-by: Paul Wankadia <junyer@google.com>