Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

src/nvim/digraph.h

@@ -0,0 +1,12 @@
+#ifndef NEOVIM_DIGRAPH_H
+#define NEOVIM_DIGRAPH_H
+
+int do_digraph(int c);
+int get_digraph(int cmdline);
+int getdigraph(int char1, int char2, int meta_char);
+void putdigraph(char_u *str);
+void listdigraphs(void);
+char_u *keymap_init(void);
+void ex_loadkeymap(exarg_T *eap);
+
+#endif  // NEOVIM_DIGRAPH_H