Move dead submodules in-tree

Signed-off-by: swurl <swurl@swurl.xyz>

externals/sirit/README.md

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+Sirit
+=====
+A runtime SPIR-V assembler. It aims to ease dynamic SPIR-V code generation
+without calling external applications (like Khronos' `spirv-as`)  
+
+Its design aims to move code that does not belong in the application to the
+library, without limiting its functionality.
+
+What Sirit does for you:
+* Sort declaration opcodes
+* Handle types and constant duplicates
+* Emit SPIR-V opcodes  
+
+What Sirit won't do for you:
+* Avoid ID duplicates (e.g. emitting the same label twice)
+* Dump code to disk
+* Handle control flow
+* Compile from a higher level language
+  
+  
+It's in early stages of development, many instructions are missing since
+they are written manually instead of being generated from a file.
+
+Example
+-------
+
+```cpp
+class MyModule : public Sirit::Module {
+public:
+    MyModule() {}
+    ~MyModule() = default;
+
+    void Generate() {
+        AddCapability(spv::Capability::Shader);
+        SetMemoryModel(spv::AddressingModel::Logical, spv::MemoryModel::GLSL450);
+        
+        auto main_type{TypeFunction(TypeVoid())};
+        auto main_func{OpFunction(TypeVoid(), spv::FunctionControlMask::MaskNone, main_type)};
+        AddLabel(OpLabel());
+        OpReturn();
+        OpFunctionEnd();
+
+        AddEntryPoint(spv::ExecutionModel::Vertex, main_func, "main");
+    }
+};
+
+// Then...
+
+MyModule module;
+module.Generate();
+
+std::vector<std::uint32_t> code{module.Assemble()};
+```