chore: enclose everything in namespace matar

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

src/cpu/cpu.cc

@@ -1,6 +1,7 @@
 #include "cpu/cpu.hh"
 #include "cpu-impl.hh"
 
+namespace matar {
 Cpu::Cpu(const Bus& bus) noexcept
   : impl(std::make_unique<CpuImpl>(bus)){};
 
@@ -10,3 +11,4 @@ void
 Cpu::step() {
     impl->step();
 };
+}