api: Rename dict_set_value to dict_set_var

Reasonings:
1. It is not used for anything, but scope dictionaries currenly. So there is no 
   need to generalize and split it into dict_set_var (which will contain some 
   scope-dictionary-specific checks) and dict_set_value (which will work for any 
   dictionary).
2. Check for key size is no longer valid for non-scope dictionaries: you *can* 
   use empty keys there. In scope dictionaries also, but you actually are not 
   supposed to store there anything, but variables.

Note that actually one may still do

    let b:[''] = 1

and “bypass” check for variable name. It won’t change what `echo b:` will show, 
but it may affect code which iterates over scope dictionary keys and sets them 
to something (if there is such code).

src/nvim/api/private/helpers.c

@@ -98,7 +98,7 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
   return vim_to_object(&di->di_tv);
 }
 
-/// Set a value in a dict. Objects are recursively expanded into their
+/// Set a value in a scope dict. Objects are recursively expanded into their
 /// vimscript equivalents.
 ///
 /// @param dict The vimscript dict
@@ -109,8 +109,8 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
 /// @param retval If true the old value will be converted and returned.
 /// @param[out] err Details of an error that may have occurred
 /// @return The old value if `retval` is true and the key was present, else NIL
-Object dict_set_value(dict_T *dict, String key, Object value, bool del,
-                      bool retval, Error *err)
+Object dict_set_var(dict_T *dict, String key, Object value, bool del,
+                    bool retval, Error *err)
 {
   Object rv = OBJECT_INIT;
 
@@ -120,7 +120,7 @@ Object dict_set_value(dict_T *dict, String key, Object value, bool del,
   }
 
   if (key.size == 0) {
-    api_set_error(err, Validation, _("Empty dictionary keys aren't allowed"));
+    api_set_error(err, Validation, _("Empty variable names aren't allowed"));
     return rv;
   }
 
@@ -129,7 +129,7 @@ Object dict_set_value(dict_T *dict, String key, Object value, bool del,
     return rv;
   }
 
-  dictitem_T *di = dict_find(dict, (uint8_t *)key.data, (int)key.size);
+  dictitem_T *di = dict_find(dict, (char_u *)key.data, (int)key.size);
 
   if (del) {
     // Delete the key