General: Convert multiple namespace specifiers to nested namespace specifiers where applicable

Makes namespacing a little less noisy

src/common/memory_pool.cpp

@@ -8,8 +8,7 @@
 
 #include "common/memory_pool.h"
 
-namespace Dynarmic {
-namespace Common {
+namespace Dynarmic::Common {
 
 Pool::Pool(size_t object_size, size_t initial_pool_size) : object_size(object_size), slab_size(initial_pool_size) {
     AllocateNewSlab();
@@ -42,6 +41,4 @@ void Pool::AllocateNewSlab() {
     remaining = slab_size;
 }
 
-
-} // namespace Common
-} // namespace Dynarmic
+} // namespace Dynarmic::Common