Sanitizers are used to modify the wrote bytes from the templates, and from
the input data. The [[infn]] method is dedicated to read the bytes coming
from outside of the template delimiters, while the [[outfn]] from the inside
of them.

A common use case is to prevent code injection coming from the data. By
example, the Html sanitizer read the Html code from the [[outfn]],
update its internal state, and then pass-through the bytes. This way the
sanitizer can correctly sanitizes bytes coming [[infn]], depending on
the target Html code location.
