fix(emacs/lean-mode.el): remove quotation marks in lean-execute

Close #294
feat(frontends/lean): add 'record' as an alias for 'structure' command
fix(frontends/lean/structure_cmd): error parsing structure without parameters followed by ': Type'
fix(frontends/lean/structure_cmd): accept ': Type' when universe levels are not specified
fix(frontends/lean/structure_cmd): bad test output
test(tests/lean/run): add structure command test
feat(frontends/lean): add 'print fields' command
feat(frontends/lean/structure_cmd): allow inheritance from two identical structures, closes #296