This way we take advantage of the constant-time insertion and removal of
elements at the beginning of Front Extensible sequences, such as list.
This implementation is closer to the original.
According to reference, insert_range should work for any Extensible
sequence or Extensible Associative sequence, but the default
implementation of insert_range_impl assumes a front_inserter is defined
for the given sequence, but neither Extensible nor every Extensible
Associative sequences are required to also be a Front Extensible
sequence. This fix rely only on insert, which is defined for every
Extensible sequence.
