PPTC & Pool Enhancements. (#1968)

* PPTC & Pool Enhancements.

* Avoid buffer allocations in CodeGenContext.GetCode(). Avoid stream allocations in PTC.PtcInfo.

Refactoring/nits.

* Use XXHash128, for Ptc.Load & Ptc.Save, x10 faster than Md5.

* Why not a nice Span.

* Added a simple PtcFormatter library for deserialization/serialization, which does not require reflection, in use at PtcJumpTable and PtcProfiler; improves maintainability and simplicity/readability of affected code.

* Nits.

* Revert #1987.

* Revert "Revert #1987."

This reverts commit 998be765cf.

ARMeilleure/State/ExecutionMode.cs

@@ -1,6 +1,6 @@
 namespace ARMeilleure.State
 {
-    enum ExecutionMode
+    enum ExecutionMode : int
     {
         Aarch32Arm = 0,
         Aarch32Thumb = 1,