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/globals.h

@@ -474,10 +474,6 @@ EXTERN buf_T *curbuf INIT( = NULL);    // currently active buffer
 #define FOR_ALL_BUF_WININFO(buf, wip) \
   for ((wip) = (buf)->b_wininfo; (wip) != NULL; (wip) = (wip)->wi_next)   // NOLINT
 
-// Iterate through all the signs placed in a buffer
-#define FOR_ALL_SIGNS_IN_BUF(buf, sign) \
-  for ((sign) = (buf)->b_signlist; (sign) != NULL; (sign) = (sign)->se_next)   // NOLINT
-
 // List of files being edited (global argument list).  curwin->w_alist points
 // to this when the window is using the global argument list.
 EXTERN alist_T global_alist;    // global argument list