Improve performance of String.concat for seq of type array or list

[abelbraaksma]

This follows the findings and timings shown in this comment: #9390 (comment)

Basically: special-casing the input seq for array gives 2x perf gain and less 2-3x less mem use, and special-casing for list gives 20% gain and 30% less mem and GC pressure.

@KevinRansom: this is the one we discussed that shows no improvement by using array-based manipulation if the input is a seq, and since String.Join uses buffered-SB internally, that may well be the reason it already performed so well.

It's also a showcase of the internal ArrayEnumerator of arrays-turned-into-sequences, however fast it is, still adding a lot of time for the indirection, hence the success of the special-casing of array. Try this new method with Array.toSeq x and with implicit x :> seq<_> and watch the difference (a factor 2).

This PR is dependent on #9481.

[cartermp]

Some conflicts and stylistic change, but otherwise this looks good!

[abelbraaksma]

Since we now have three execution paths (for list, seq and array), I changed the tests to execute all three paths for the existing tests 9c6ba3f.

[abelbraaksma]

Strange, it keeps saying "Changes requested" even though all requests have been resolved. Maybe I'm doing something wrong...

[KevinRansom]

@abelbraaksma , the requester has to acknowledge that they have been handled I believe.

[abelbraaksma]

All requested changes have been implemented and build is green. @cartermp / @KevinRansom, is it ok like this?

[abelbraaksma]

@cartermp, you merged master into this, I think this is now ready?

[cartermp]

@abelbraaksma I think this is ready, but this will require one other review (preferably @KevinRansom since he previously requested changes)

[abelbraaksma]

/cc @KevinRansom

[abelbraaksma]

Thanks @KevinRansom!