Avoid unused variable warnings for return values used only in assertions

[chrisnovakovic]

The return values of many functions are assigned to variables for the sole purpose of using them in assertion conditions, which causes unused variable warnings to be emitted at compile-time when NDEBUG is defined. Use some compiler trickery in the definition of rd_assert() to avoid unused variable warnings when variables in the assertion condition are only used for that purpose, and ensure that assertions whose conditions contain single-use variables use rd_assert() instead of assert().