feat(frontends/lean): nicer notation for dependent if-then-else
feat(library/definitional): add "datastructure" for storing recursive equations
feat(library/definitional/equations): add support for serializing equations
feat(frontends/lean): parse "decreasing" expressions
chore(library/definitional/equations): fix style
refactor(frontends/lean/decl_cmds): cleanup definition_cmd
feat(frontends/lean): parse recursive equations