Investigate whether we should increase the default nursery size

[rolfbjarne]

Our default nursery size is 512k:

xamarin-macios/tools/common/Application.cs

Line 1309 in 887e67c

string ret = "nursery-size=512k";

We've had this default since 2012 (https://github.com/xamarin/maccore/commit/40d3775d9028edf0292ca625e30c1c3647b391d6), and in the meantime devices have a lot more memory and they're much faster too. The consequence of a small nursery is that there will be more frequent garbage collections, and this can slow down memory allocations (the consequence of a bigger nursery is longer GC pauses).

So it might be time to investigate whether we should increase the default.

@ivanpovazan suggested 4MB, but we should measure this (the most sensitive/visible part is typically dropped frames while scrolling a list/grid view).

We should also add an MSBuild property to control the nursery size, instead of having to put it in MtouchExtraArgs.