Splay Tree Improvements

Splay tree insert function which takes Node, now performs non-GC memory allocation (function not yet annotated @nogc) using an arena/region strategy with enough room initially reserved for 65536 nodes, backed by malloc.

Splay tree now probabalistically splays , which increases speed up to 10-20% on certain serial query workloads. To do, we will parameterize this (including, off course, 100% probability).

API unchanged