feat(shell/lean): add '--server-trace' flag, closes #264
fix(frontends/lean/elaborator): remove invalid assertions

These assertions became invalid when we changed the behavior of undef
indentifiers at

https://github.com/leanprover/lean/commit/8e6de933948748b26ff685bedb76f3e691df9e36
fix(frontends/lean/elaborator): remove invalid assertions