feat(api): add nvim__redraw for more granular redrawing

Experimental and subject to future changes.
Add a way to redraw certain elements that are not redrawn while Nvim is waiting
for input, or currently have no API to do so. This API covers all that can be
done with the :redraw* commands, in addition to the following new features:
- Immediately move the cursor to a (non-current) window.
- Target a specific window or buffer to mark for redraw.
- Mark a buffer range for redraw (replaces nvim__buf_redraw_range()).
- Redraw the 'statuscolumn'.

src/nvim/math.c

@@ -77,6 +77,23 @@ int xctz(uint64_t x)
 #endif
 }
 
+/// Count number of set bits in bit field.
+int popcount(uint64_t x)
+{
+  // Use compiler builtin if possible.
+#if defined(__clang__) || defined(__GNUC__)
+  return __builtin_popcountll(x);
+#else
+  int count = 0;
+  for (; x != 0; x >>= 1) {
+    if (x & 1) {
+      count++;
+    }
+  }
+  return count;
+#endif
+}
+
 /// For overflow detection, add a digit safely to an int value.
 int vim_append_digit_int(int *value, int digit)
 {