Kernel: Add KString, a single-owner string with OOM failure exposion

This is a simple string class for use in the kernel. It encapsulates
a length + character array in a single-allocation object.

Main differences from AK::String:

- Single-owner (no reference counting.)
- Allocation failures are exposed, not hidden.

The basic idea is to allow better and more precise string management
in the kernel.

Kernel/CMakeLists.txt

@@ -112,6 +112,7 @@ set(KERNEL_SOURCES
     Interrupts/SpuriousInterruptHandler.cpp
     Interrupts/UnhandledInterruptHandler.cpp
     KBufferBuilder.cpp
+    KString.cpp
     KSyms.cpp
     Lock.cpp
     Net/E1000NetworkAdapter.cpp