refactor(util/trie): modify interface to avoid the creation of many temporary optional values and inc/dec reference counters

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/token_set): register builtin commands

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/token_set): move to frontends/lean

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add new scanner

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