kernel: some stringobj tweaks

* make MakeImmutableString static line
* remove unused `IsStringConvFilt`
* rewrite MakeString to not use C_NEW_STRING

Author: fingolfin

src/stringobj.c

@@ -1368,8 +1368,6 @@ void ConvString (
 **  otherwise.   If <obj> is a  string it  changes  its representation to the
 **  string representation.
 */
-Obj IsStringConvFilt;
-
 Int IsStringConv (
     Obj                 obj )
 {
@@ -1388,17 +1386,6 @@ Int IsStringConv (
 }
 
 
-/****************************************************************************
-**
-*F  MakeImmutableString(  <str> ) make a string immutable in place
-**
-*/
-
-void MakeImmutableString( Obj str )
-{
-    RetypeBag(str, IMMUTABLE_TNUM(TNUM_OBJ(str)));
-}
-
 Obj ConvImmString(Obj str)
 {
   Obj result;

src/stringobj.h

@@ -321,13 +321,15 @@ extern Int IsStringConv (
     memcpy( CHARS_STRING(string), (cstr), tmp_len ); \
   } while ( 0 );
 
+
 /****************************************************************************
 **
-*F  MakeImmutableString(  <str> ) make a string immutable in place
-**
+*F  MakeImmutableString( <str> ) . . . . . . make a string immutable in place
 */
-
-void MakeImmutableString(Obj str);
+static inline void MakeImmutableString(Obj str)
+{
+    RetypeBag(str, IMMUTABLE_TNUM(TNUM_OBJ(str)));
+}
 
 
 // Functions to create mutable and immutable GAP strings from C strings.
@@ -336,15 +338,16 @@ void MakeImmutableString(Obj str);
 
 static inline Obj MakeString(const Char * cstr)
 {
-    Obj result;
-    C_NEW_STRING(result, strlen(cstr), cstr);
+    size_t len = strlen(cstr);
+    Obj    result = NEW_STRING(len);
+    memcpy(CHARS_STRING(result), cstr, len);
     return result;
 }
 
 static inline Obj MakeImmString(const Char * cstr)
 {
     Obj result = MakeString(cstr);
-    RetypeBag(result, IMMUTABLE_TNUM(T_STRING));
+    MakeImmutableString(result);
     return result;
 }