btree has conversion data loss warning from `size_t` to `const unsigned char` conversion

**Describe the bug**

Upgraded to use latest LTS: 20220623.0 (upgraded from 20210324.2), and btree now fails to compile on windows with a significant numeric truncation issue. (I believe this is distinct from #40 as a general / widespread concern)

**Steps to reproduce the bug**

See github gist: https://gist.github.com/matthauck/011a5b8bcdbd8fafb5440e46b7561703

**What version of Abseil are you using?**

20220623.0

**What operating system and version are you using**

Windows 10 Pro

**What compiler and version are you using?**

Microsoft Visual Studio Professional 2019
Version 16.11.7

**What build system are you using?**

cmake 3.19.4

**Additional context**

N/A.