fix(init): initialization problem
feat(frontends/lean/parse_table): add get_head_index auxiliary function for indexing notation declarations
feat(frontends/lean/parse_config): add get_notation_entries auxiliary function for returning the list of notation declarations that start with a given head symbol

This API is need to take notation declarations into account when pretty
printing expressions.
refactor(frontends/lean/inductive_cmd): move auxiliary method to expr.h
fix(library/type): modify declaration order and make sure Type1, Type2 and Type3 notations have precedence over Type' during pretty printing
fix(library/module): bug in import_module