feat(userreg): Add user-defined registers to Neovim.

This change unlocks additional registers for Neovim by allowing a user
to define their own behavior for non-builtin registers.

This is accopmlished through a new option 'userregfunc'

The 'userregfunc' defines the function to call when handling a register
for which there is no builtin functionality.

The 'userregfunc' function should take 3 arguments:

   action - Either "yank" or "put"
   register - The character corresponding to the register
   content - In the case of action == "yank", the dictionary describing
             the yanked content, with the following keys:

             {type} - Either "char", "line" or "block"
             {lines} - The lines being yanked as a list
             {width} - The width in case of "block" mode.
             {additional_data} - Additional data (can be returned in
             "put" mode)

   In case of "put" this function should return the content to put. This
   content can be either:

     * A dictionary in the same template as content above.
     * A list of strings. This will be assumed to be "line" mode.
     * A string. This will be assumed to be "char" mode.

An example of a "null" 'userregfunc' that provides an implementation
identical to traditional vim registers would be:

      let s:contents = {}
      function! MyUserregFunction(action, register, content) abort
         if a:action == "put"
           return get(s:contents, a:register, "")
        else
           let s:contents[a:register] = a:content
        endif
      endfunction
      set userregfun=MyUserregFunction

  It is important to note that any valid unicode character can now be a
  register, including something like @☺.

  This change also addresses the multibyte parsing issues surrounding

  let @a = 'xyz'
  let @🔨 = 'hammer'

1 files changed, 42 insertions, 0 deletions

diff --git a/src/nvim/yankmap.c b/src/nvim/yankmap.c
new file mode 100644
index 0000000000..d9229e015d
--- /dev/null
+++ b/src/nvim/yankmap.c
@@ -0,0 +1,42 @@
+#include "nvim/yankmap.h"
+
+#include "nvim/memory.h"
+
+void init_yankmap(yankmap_T* map)
+{
+  memset(map, 0, sizeof(yankmap_T));
+
+  map_init(int, ptr_t, &map->reg_to_yankreg);
+  map_init(ptr_t, int, &map->yankreg_to_reg);
+}
+
+yankreg_T* yankmap_get(yankmap_T* yankmap, int reg)
+{
+  yankreg_T** ret =
+    (yankreg_T**) map_ref(int, ptr_t)(&yankmap->reg_to_yankreg, reg, true);
+
+  if (ret) {
+    if (! (*ret)) {
+      *ret = xcalloc(sizeof(yankreg_T), 1);
+    }
+
+    /* Add the back-reference */
+    int* ref = map_ref(ptr_t, int)(&yankmap->yankreg_to_reg, *ret, true);
+    *ref = reg;
+
+    return *ret;
+  }
+
+  return NULL;
+}
+
+int yankmap_find(yankmap_T* yankmap, yankreg_T* yankreg)
+{
+  int* ref = map_ref(ptr_t, int)(&yankmap->yankreg_to_reg, yankreg, false);
+
+  if (ref) {
+    return *ref;
+  }
+
+  return -1;
+}