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

ARMeilleure/CodeGen/X86/IntrinsicInfo.cs

@@ -1,6 +1,6 @@
 namespace ARMeilleure.CodeGen.X86
 {
-    struct IntrinsicInfo
+    readonly struct IntrinsicInfo
     {
         public X86Instruction Inst { get; }
         public IntrinsicType  Type { get; }