Add new marktree data structure for storing marks

This is inspired by Atom's "marker index" data structure to efficiently
adjust marks to text insertions deletions, but uses a wide B-tree
(derived from kbtree) to keep the nesting level down.

src/nvim/marktree.h

@@ -0,0 +1,76 @@
+#ifndef NVIM_MARKTREE_H
+#define NVIM_MARKTREE_H
+
+#include <stdint.h>
+#include "nvim/map.h"
+#include "nvim/garray.h"
+
+#define MT_MAX_DEPTH 20
+#define MT_BRANCH_FACTOR 10
+
+typedef struct {
+  int32_t row;
+  int32_t col;
+} mtpos_t;
+
+typedef struct {
+  int32_t row;
+  int32_t col;
+  uint64_t id;
+  bool right_gravity;
+} mtmark_t;
+
+typedef struct mtnode_s mtnode_t;
+typedef struct {
+  int oldcol;
+  int i;
+} iterstate_t;
+
+typedef struct {
+  mtpos_t pos;
+  int lvl;
+  mtnode_t *node;
+  int i;
+  iterstate_t s[MT_MAX_DEPTH];
+} MarkTreeIter;
+
+
+// Internal storage
+//
+// NB: actual marks have id > 0, so we can use (row,col,0) pseudo-key for
+// "space before (row,col)"
+typedef struct {
+  mtpos_t pos;
+  uint64_t id;
+} mtkey_t;
+
+struct mtnode_s {
+  int32_t n;
+  int32_t level;
+  // TODO(bfredl): we could consider having a only-sometimes-valid
+  // index into parent for faster "chached" lookup.
+  mtnode_t *parent;
+  mtkey_t key[2 * MT_BRANCH_FACTOR - 1];
+  mtnode_t *ptr[];
+};
+
+// TODO(bfredl): the iterator is pretty much everpresent, make it part of the
+// tree struct itself?
+typedef struct {
+  mtnode_t *root;
+  size_t n_keys, n_nodes;
+  uint64_t next_id;
+  // TODO(bfredl): the pointer to node could be part of the larger
+  // Map(uint64_t, ExtmarkItem) essentially;
+  PMap(uint64_t) *id2node;
+} MarkTree;
+
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "marktree.h.generated.h"
+#endif
+
+#define MARKTREE_PAIRED_FLAG (((uint64_t)1) << 1)
+#define MARKTREE_END_FLAG (((uint64_t)1) << 0)
+
+#endif  // NVIM_MARKTREE_H