lib: create lib `f-plus`

# Description

The initial version of `futils` contains a lot of "core stuff" that will not be included in the `f-core` lib. One could create a `f-plus` lib to work as a complement to `f-core`, containing part of that "core stuff".

The lib should import `f-core` and:

1. add a class `plus.Specs/s` with additional specs
2. add the class `plus.Funcs/f` with some `TypedFuncs`