Support key param for sortBy and sortByDescending

[couzic]

Currently, it is possible to sort using the key of an item, but only through the Comparator and ComparatorFactory interfaces, for example:

sequency/test/sorted.test.ts

Line 67 in 7be941d

.sorted(it => it.compareBy("x"))

sequency/test/sorted.test.ts

Line 89 in 7be941d

.sorted(it => it.compareByDescending("x"))

sequency/test/sorted.test.ts

Line 108 in 7be941d

.thenBy("y"))

sequency/test/sorted.test.ts

Line 135 in 7be941d

.thenByDescending("x"))

The goal of this PR is to enable sorting by key directly on the Sequence type, like:

        sequenceOf(a4, a1, a3, a23)
            .sortedBy("a")

        sequenceOf(a4, a1, a3, a23)
            .sortedByDescending("a")

Also, this PR exports the Comparator and ComparatorFactory types, so that they are added to the docs (which suppresses a couple warnings when generating them)

[couzic]

Hello @winterbe,
I've been willing to do this for a long time. With this PR, the sorting functions should finally be consistent.