Restructure os code

base.kl

@@ -0,0 +1,24 @@
+Os :: #import "os_windows.kl"
+
+SizeU :: #strict U64
+
+Arena :: struct
+  memory: Os.Memory
+  alignment: U64
+  len: U64
+
+clamp_top_sizeu :: (val: SizeU, max: SizeU): SizeU
+  if val > max
+    return max
+  return val
+
+get_align_offset :: (size: SizeU, align: SizeU): SizeU
+  mask := align - 1
+  val  := size & mask
+  if val != 0
+    val = align - val
+  return val
+
+align_up :: (size: SizeU, align: SizeU): SizeU
+  result := size + get_align_offset(size, align)
+  return result