Show Java translation, multi-line expressions, bug fixes

1. Change source in the generated Java file to have a file name instead of the actual Frege source.
2. When a module is submitted on online REPL, do not just append imports (to block stdout, stderr) to the source which can
cause issues when the module is closed already with braces instead after parsing, add import
definitions to the parsed declarations.
3. Support multi-line expressions
4. Upgrade Frege version to 3.21.297 g6b54457
5. A Java proxy to easily implement interfaces in Frege