feat(frontends/lean/builtin_cmds): add 'variables' command family

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(frontends/lean): expose notation_entry and token_entry structures, and add functions for parsing notation without affecting the environment

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): add 'flag' for disabling 'unknown identifier' errors

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/expr_lt): add lex comparison for expression pairs

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/name): add name_pair, and lex order

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add calculational proof environment extension, it stores transitivity, reflexivity (and substitution) rules

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>