Make structs readonly when applicable (#4002)

* Make all structs readonly when applicable. It should reduce amount of needless defensive copies

* Make structs with trivial boilerplate equality code record structs

* Remove unnecessary readonly modifiers from TextureCreateInfo

* Make BitMap structs readonly too

Ryujinx.HLE/HOS/Kernel/Common/KernelInit.cs

@@ -5,7 +5,7 @@ namespace Ryujinx.HLE.HOS.Kernel.Common
 {
     static class KernelInit
     {
-        private struct MemoryRegion
+        private readonly struct MemoryRegion
         {
             public ulong Address { get; }
             public ulong Size    { get; }

Ryujinx.HLE/HOS/Kernel/Common/OnScopeExit.cs

@@ -2,7 +2,7 @@
 
 namespace Ryujinx.HLE.HOS.Kernel.Common
 {
-    struct OnScopeExit : IDisposable
+    readonly struct OnScopeExit : IDisposable
     {
         private readonly Action _action;
         public OnScopeExit(Action action) => _action = action;