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

Makes namespacing a little less noisy

src/frontend/decoder/decoder_detail.h

@@ -14,8 +14,7 @@
 #include "common/bit_util.h"
 #include "common/mp.h"
 
-namespace Dynarmic {
-namespace Decoder {
+namespace Dynarmic::Decoder {
 namespace detail {
 
 /**
@@ -163,5 +162,4 @@ public:
 };
 
 } // namespace detail
-} // namespace Decoder
-} // namespace Dynarmic
+} // namespace Dynarmic::Decoder

src/frontend/decoder/matcher.h

@@ -10,8 +10,7 @@
 
 #include "common/assert.h"
 
-namespace Dynarmic {
-namespace Decoder {
+namespace Dynarmic::Decoder {
 
 /**
  * Generic instruction handling construct.
@@ -75,5 +74,4 @@ private:
     handler_function fn;
 };
 
-} // namespace Decoder
-} // namespace Dynarmic
+} // namespace Dynarmic::Decoder