perf(column): only build fold/sign column when present in 'statuscolumn'

Problem:    The fold and sign column is built and stored regardless of
            whether the corresponding item is present in 'statuscolumn'.
Solution:   Since the 'statuscolumn' parses itself, we can defer
            building the columns until the corresponding item is
            actually encountered.

src/nvim/fold_defs.h

@@ -0,0 +1,17 @@
+#ifndef NVIM_FOLD_DEFS_H
+#define NVIM_FOLD_DEFS_H
+
+#include "nvim/pos.h"
+
+// Info used to pass info about a fold from the fold-detection code to the
+// code that displays the foldcolumn.
+typedef struct foldinfo {
+  linenr_T fi_lnum;             // line number where fold starts
+  int fi_level;                 // level of the fold; when this is zero the
+                                // other fields are invalid
+  int fi_low_level;             // lowest fold level that starts in the same
+                                // line
+  linenr_T fi_lines;
+} foldinfo_T;
+
+#endif  // NVIM_FOLD_DEFS_H