Revert "series: in the generic ctor, use perfect forwarding of members instead of different functions."

This reverts commit 1c2803297adcbe64d709ff4b7693a511c5cdc1cd.
Revert "series: beautify and simplify some internal functions related to term insertion."

This reverts commit 1d0365a7107f266fc355099212af167883a3b1c6.
Fix some missing includes and simplify a method in series.