Remove indirection of IArray operations

[nicolasstucki]

Part of #11298

[odersky]

It would be good to do some micro-benchmarks about performance of array operations vs IArray operations. Ideally the performance should be the same after warmup. We should test direct access with apply as well as common collection operations such as map, flatMap, filter, foldLeft. And we should test with elements of both primitive and reference types.

[nicolasstucki]

This change will not really help with performance. It only makes the module initialization a bit sorter.

Both #11319 and #11329 take advantage of this change on a semantic level as it prioritizes these extension methods over implicit conversion.

[nicolasstucki]

This also removes the indirection on the docs