This commit brings following changes and fixes:
- Implement parseExpr and compileParsedExpr;
- Fix compileExpr and dynCompilerExpr, which returned () for empty expr;
- Fix :def and :cmd, which didn't work if IO or String is not in scope;
- Use GHCiMonad instead IO in :def and :cmd;
- Clean PrelInfo: delete dead comment and duplicate entries, add assertion.
See new tests for more details.