Resolve by expression (ap_expr)

Resolving variables by expression could be an alternative for resolution by executing a process for simple conditionals or copying of variables. The functionality would overlap with SetEnvIfExpr of the mod_setenvif module and RewriteRule (when is this expr evaluated?) of the mod_rewrite module, but would be consistent with ResolveByProcess and would offer further functionality such as evaluation at different stages of the request lifecycle.