`sanity_check` methods throughout the codebase shouldn't be part of the API

Actually I'll just file an issue. There are a lot of sanity_check methods in the code, many of which are not called from within the library, and at least one has a doc example suggesting to use it to check "whether all spend paths are accessible in the Bitcoin network".

So we need to re-assess all these functions and see if they can be pulled into the type system somehow so you simply can't create the objects without running the (non-pub) methods.

Originally posted by @apoelstra in https://github.com/rust-bitcoin/rust-miniscript/pull/652#discussion_r1514517762

Comienza inventariando los métodos sanity_check en todo el código y revisando la discusión vinculada del pull request, incluido el ejemplo de la documentación sobre las rutas de gasto de Bitcoin. Determina qué métodos se pueden invocar externamente y qué invariantes debería imponer en su lugar el sistema de tipos; se considerará terminado cuando la API ya no exponga comprobaciones inapropiadas y la validez de los objetos se gestione de forma coherente.