simplify initLeaf step of the writeLeaves folder function
experimental change.  make LeafState contain mutables and use Seq.iter instead of Seq.fold.
Revert "experimental change.  make LeafState contain mutables and use Seq.iter instead of Seq.fold."

This reverts commit 9d2baaa90c24e06d5e8a8537b0fc1e1d9039a95f.
notes
switch all the f# stuff to net40 and to get FSharp.Core from nuget instead of from whatever is installed with xamarin.  this makes it possible to use fsharpx.collections as a nuget package without quirky problems instead of copy/paste code from same into lsm.fs.
another attempt to convert PageBuilder.Write to use async.  this attempt is faster than the previous one, but still 70% slower than just blocking on synchronous writes.  this attempt converts both LeafState and ParentState to use mutables, then a simple loop over the items instead of using Seq/List.fold.  I also made some effort to make sure asyncs don't get created for cases where they are not needed, such as adding a non-overflowing kvp to a leaf.  still kinda slow.