iOS/macOS

• Fix nil string bug while inserting empty string.
• Reduce size of binary and provide a non-bitcode scheme by --configuration WithoutBitcode for Carthage.
• Avoid conflict between builtin SQLCipher and other SQLite based library(including system library).
• Code template is not installed automatically now. Developers can run curl https://raw.githubusercontent.com/Tencent/wcdb/master/tools/templates/install.sh -s | sh to install it manually.