Assemble uint32_t instead of uint8_t

Vulkan receives SPIR-V modules with a uint32_t alignment. Returning
uint8_t forced users to invoke undefined behaviour (reinterpret_cast)
or copy.

src/sirit.cpp

@@ -24,8 +24,8 @@ Module::Module(u32 version) : version(version) {}
 
 Module::~Module() = default;
 
-std::vector<u8> Module::Assemble() const {
-    std::vector<u8> bytes;
+std::vector<u32> Module::Assemble() const {
+    std::vector<u32> bytes;
     Stream stream{bytes};
 
     stream.Write(spv::MagicNumber);