`cordyceps::List` docs for `push_*`/`pop_*` should claim constant time

[dataphract]

The docs for push_front(), push_back(), pop_front() and pop_back() all have the following:

This operation should compute in O(n) time.

As best I can tell from reading the source, these are all constant-time operations -- the methods traverse at most two nodes.

[hawkw]

Whoops, I think this was a copy-paste mishap --- good catch!