sort --merge works, but ignores --unique and --reverse

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.

tests/fixtures/sort/merge_ints_interleaved.expected

@@ -0,0 +1,9 @@
+1
+2
+3
+4
+5
+6
+7
+8
+9

tests/fixtures/sort/merge_ints_interleaved_1.txt

@@ -0,0 +1,3 @@
+1
+4
+7

tests/fixtures/sort/merge_ints_interleaved_2.txt

@@ -0,0 +1,3 @@
+2
+5
+8

tests/fixtures/sort/merge_ints_interleaved_3.txt

@@ -0,0 +1,3 @@
+3
+6
+9