%merge: let mergers record a typing-symbol, rather than a type

Symbols are richer than types, and in M4 it is my simpler (and more
common) to deal with symbols rather than types.  So let's associate
mergers to a symbol rather than a type name.

* src/reader.h (merger_list): Replace the 'type' member by a symbol
member.
* src/reader.c (record_merge_function_type): Take a symbol as
argument, rather than a type name.
* src/output.c (merger_output): Adjust.

src/reader.h

@@ -31,7 +31,10 @@ typedef struct merger_list
 {
   struct merger_list* next;
   uniqstr name;
-  uniqstr type;
+  /* One symbol whose type is the one used by all the symbols on which
+     this merging function is used.  */
+  symbol *sym;
+  /* Where SYM was bound to this merging function.  */
   location type_declaration_loc;
 } merger_list;