Move ArrayPool to System.Private.CoreLib

[JeremyKuhne]

Copy ArrayPool source from CoreFX and expose in System.Private.CoreLib.

This is part of facilitating the move of FileStream.
https://github.com/dotnet/corefx/issues/12810

@ianhays, @stephentoub, @sokket, @jkotas

[JeremyKuhne]

@dotnet-bot Test this please (Jenkins rebooted)

[ianhays]

Were any changes to the CoreFX sources required or did you just have to copy-paste those?

The build stuff LGTM.

[ericstj]

It looks ok to me. In corefx we'll want to cross-compile buffers for netcoreapp1.1 as a partial facade.

[JeremyKuhne]

@ianhays This was just a straight copy, except for the string resource.

[jamesqo]

Awesome! We should definitely look into using this for the default Stream.CopyTo / CopyToAsync impl.

[jkotas]

@Maoni0 The ArrayPool is part of CoreLib now .... it may be interesting to look into whether it would be worth it to make GC aware of it, to get better perf characteristics.

[Maoni0]

thanks Jan. @vancem did a pinnable buffer pool implementation. I can see some of the heuristics being used in this buffer pool as impl well. it goes the other way - it's aware of GCs happening; we can look at it from that direction first.

@swgillespie @adityamandaleeka