api: Add FUNC_ATTR_DEFERRED attribute to a number of functions

Any function that can directly mutate the screen or execute vimscript had the
attribute applied.

src/nvim/api/buffer.c

@@ -69,6 +69,7 @@ String buffer_get_line(Buffer buffer, Integer index, Error *err)
 /// @param line The new line.
 /// @param[out] err Details of an error that may have occurred
 void buffer_set_line(Buffer buffer, Integer index, String line, Error *err)
+  FUNC_ATTR_DEFERRED
 {
   Object l = STRING_OBJ(line);
   Array array = {.items = &l, .size = 1};
@@ -81,6 +82,7 @@ void buffer_set_line(Buffer buffer, Integer index, String line, Error *err)
 /// @param index The line index
 /// @param[out] err Details of an error that may have occurred
 void buffer_del_line(Buffer buffer, Integer index, Error *err)
+  FUNC_ATTR_DEFERRED
 {
   Array array = ARRAY_DICT_INIT;
   buffer_set_line_slice(buffer, index, index, true, true, array, err);
@@ -163,6 +165,7 @@ void buffer_set_line_slice(Buffer buffer,
                       Boolean include_end,
                       ArrayOf(String) replacement,
                       Error *err)
+  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -314,6 +317,7 @@ Object buffer_get_var(Buffer buffer, String name, Error *err)
 /// @param[out] err Details of an error that may have occurred
 /// @return The old value
 Object buffer_set_var(Buffer buffer, String name, Object value, Error *err)
+  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -349,6 +353,7 @@ Object buffer_get_option(Buffer buffer, String name, Error *err)
 /// @param value The option value
 /// @param[out] err Details of an error that may have occurred
 void buffer_set_option(Buffer buffer, String name, Object value, Error *err)
+  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -399,6 +404,7 @@ String buffer_get_name(Buffer buffer, Error *err)
 /// @param name The buffer name
 /// @param[out] err Details of an error that may have occurred
 void buffer_set_name(Buffer buffer, String name, Error *err)
+  FUNC_ATTR_DEFERRED
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
@@ -444,6 +450,7 @@ void buffer_insert(Buffer buffer,
                    Integer lnum,
                    ArrayOf(String) lines,
                    Error *err)
+  FUNC_ATTR_DEFERRED
 {
   buffer_set_line_slice(buffer, lnum, lnum, false, true, lines, err);
 }