[kernel] Rename stack_cache to buffer_cache

Renamed and genericized the stack_cache class to manage any address
range area of buffers or memory regions. Removed singleton and created
some globals that map to different address regions (kernel stacks,
kernel buffers).

Tags: vmem virtual memeory

modules.yaml

@@ -46,7 +46,7 @@ modules:
             - src/kernel/scheduler.cpp
             - src/kernel/screen.cpp
             - src/kernel/serial.cpp
-            - src/kernel/stack_cache.cpp
+            - src/kernel/buffer_cache.cpp
             - src/kernel/symbol_table.cpp
             - src/kernel/syscall.cpp
             - src/kernel/syscall.s