refactor(sign): move legacy signs to extmarks

Problem:  The legacy signlist data structures and associated functions are
          redundant since the introduction of extmark signs.
Solution: Store signs defined through the legacy commands in a hashmap, placed
          signs in the extmark tree. Replace signlist associated functions.

Usage of the legacy sign commands should yield no change in behavior with the
exception of:
  - "orphaned signs" are now always removed when the line it is placed on is
    deleted. This used to depend on the value of 'signcolumn'.
  - It is no longer possible to place multiple signs with the same identifier
    in a single group on multiple lines. This will now move the sign instead.

Moreover, both signs placed through the legacy sign commands and through
|nvim_buf_set_extmark()|:
  - Will show up in both |sign-place| and |nvim_buf_get_extmarks()|.
  - Are displayed by increasing sign identifier, left to right.
    Extmark signs used to be ordered decreasingly as opposed to legacy signs.

src/nvim/main.c

@@ -70,7 +70,6 @@
 #include "nvim/quickfix.h"
 #include "nvim/runtime.h"
 #include "nvim/shada.h"
-#include "nvim/sign.h"
 #include "nvim/statusline.h"
 #include "nvim/strings.h"
 #include "nvim/syntax.h"
@@ -218,8 +217,6 @@ void early_init(mparm_T *paramp)
   TIME_MSG("inits 1");
 
   set_lang_var();               // set v:lang and v:ctype
-
-  init_signs();
 }
 
 #ifdef MAKE_LIB