Add a variant of valid_var_name which accepts const wchar_t *

This avoids creating some unnecessary strings.

src/common.cpp

@@ -1875,7 +1875,15 @@ bool valid_var_name_char(wchar_t chr) { return fish_iswalnum(chr) || chr == L'_'
 
 /// Test if the given string is a valid variable name.
 bool valid_var_name(const wcstring &str) {
-    return std::find_if_not(str.begin(), str.end(), valid_var_name_char) == str.end();
+    // Note do not use c_str(), we want to fail on embedded nul bytes.
+    return std::all_of(str.begin(), str.end(), valid_var_name_char);
+}
+
+bool valid_var_name(const wchar_t *str) {
+    for (size_t i = 0; str[i] != L'\0'; i++) {
+        if (!valid_var_name_char(str[i])) return false;
+    }
+    return true;
 }
 
 /// Test if the string is a valid function name.

src/common.h

@@ -638,6 +638,7 @@ std::string get_path_to_tmp_dir();
 
 bool valid_var_name_char(wchar_t chr);
 bool valid_var_name(const wcstring &str);
+bool valid_var_name(const wchar_t *str);
 bool valid_func_name(const wcstring &str);
 
 // Return values (`$status` values for fish scripts) for various situations.