FileMerger receives Lines Iterables of the pre-sorted input files
via push_file() It implements Iterator, which yields lines from the
input files in (merged) sorted order. If the input files are not sorted,
then the behavior is undefined.
Internally, FileMerger uses a
std::collections::BinaryHeap<MergeableFile>.
MergeableFile is an internal helper that implements Ord in a way that
BinaryHeap can use (note that we want smallest-first, but BinaryHeap
returns largest first, so MergeableFile::cmp() calls reverse() on
whatever compare_by() returns.
Made a new function sort_by(lines, compare_fns), which accepts a
list of compare_fns and calls lines.sort_by() with a closure that
calls each compare_fn in turn until one returns something other
than equal.
Default behavior ensures that String::cmp is the last element in the
compare_fns list (referred to as 'last resort' sorting by man sort).
Passing --stable (-s) turns this behaviour off.
Test cases provided for `sort --month` and `sort --month --stable`.
Instead of using numerals to denote individual cases, I used descriptive
case names. I also changed the extension for the expected output fixture
to match other tests.
I removed one redundant test and another unnecessary helper function.
