Add automatic generation of headers

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

src/nvim/edit.h

@@ -12,47 +12,10 @@
 #define CPT_INFO    3   /* "info" */
 #define CPT_COUNT   4   /* Number of entries */
 
-int edit(int cmdchar, int startln, long count);
-void edit_putchar(int c, int highlight);
-void edit_unputchar(void);
-void display_dollar(colnr_T col);
-void change_indent(int type, int amount, int round, int replaced,
-                   int call_changed_bytes);
-void truncate_spaces(char_u *line);
-void backspace_until_column(int col);
-int vim_is_ctrl_x_key(int c);
-int ins_compl_add_infercase(char_u *str, int len, int icase,
-                            char_u *fname, int dir,
-                            int flags);
-void set_completion(colnr_T startcol, list_T *list);
-void ins_compl_show_pum(void);
-char_u *find_word_start(char_u *ptr);
-char_u *find_word_end(char_u *ptr);
-int ins_compl_active(void);
-int ins_compl_add_tv(typval_T *tv, int dir);
-void ins_compl_check_keys(int frequency);
-int get_literal(void);
-void insertchar(int c, int flags, int second_indent);
-void auto_format(int trailblank, int prev_line);
-int comp_textwidth(int ff);
-int stop_arrow(void);
-void set_last_insert(int c);
-void free_last_insert(void);
-char_u *add_char2buf(int c, char_u *s);
-void beginline(int flags);
-int oneright(void);
-int oneleft(void);
-int cursor_up(long n, int upd_topline);
-int cursor_down(long n, int upd_topline);
-int stuff_inserted(int c, long count, int no_esc);
-char_u *get_last_insert(void);
-char_u *get_last_insert_save(void);
-void replace_push(int c);
-int replace_push_mb(char_u *p);
-void fixthisline(int (*get_the_indent)(void));
-void fix_indent(void);
-int in_cinkeys(int keytyped, int when, int line_is_empty);
-int hkmap(int c);
-int ins_copychar(linenr_T lnum);
+typedef int (*IndentGetter)(void);
 
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "edit.h.generated.h"
+#endif
 #endif /* NVIM_EDIT_H */