This change allows to disentangle the usages of `lib::SeveralBuilder`,
so that at any time during the build process only a single instance is
actively populated, all in one row — and thus the required storage can
either be pre-allocated, or dynamically extended and shrinked (when
filling elements into the last `SeveralBuilder` currently activated)
By packaging into a λ-closure, the building of the actual `Port`
implementation objects (≙ `Turnout` instances) is delayed until the
very end of the build process, and then unloaded into yet another
`lib::Several` in one strike. Temporarily, those building functor
objects are „hidden“ in the current stack frame, as a new `NodeBuilder`
instance is dropped off with an adapted type parameter (embedding the
λ-type produced by the last nested `PortBuilder` invocation, while
inheriting from previous ones.
However, defining a special constructor to cause this »chaining«
poses some challenge (regarding overload resolution). Moreover,
since the actual processing function shall be embedded directly
(as opposed to wrapping it into a `std::function`), further problems
can arise when this function is given as a ''function reference''
Ichthyostega
df37fec500