Enable warnings in our C code

For example [this problem](https://github.com/haskell/bytestring/pull/582#issuecomment-1522476846) with a recent patch would have been trivial to diagnose if we enabled `-Wconversion`. But enabling a reasonable warning set would cause a lot of noise from our existing C code, so this task has rather low priority.

The issue names no files or tests; start by locating the build configuration and C sources used by bytestring, then inspect the effect of -Wconversion and other reasonable warnings. Done means the warning set is enabled without overwhelming noise from existing C code.

Written by the indexing model from the issue text.