Change lookahead_list into a queue

Using builtin `commandline -f`, one would expect to have commands executed in
the order that they were given.  This motivates the change to a queue.

Unfortunately, fish internals still need lookahead_list to act as a stack.  Add
and rename functions to support both cases and have lookahead_list as
a std::deque internally.

This code is delicate, and we should probably dog-food this in nightly for
a while before the next-minor release.

Fixes #1567

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