builtin::trim: just use sv_setpvn()

This includes all the fixes and makes things simpler.

It does slightly change the taint behaviour, rather than making TARG
tainted iff source is tainted, it changes to the behaviour of the rest
of perl, making TARG tainted if any tainted input is seen in the
current expression.

builtin.c

@@ -272,26 +272,14 @@ XS(XS_builtin_trim)
         }
     }
 
-    SV_CHECK_THINKFIRST(TARG);
-    SvUPGRADE(TARG, SVt_PV);
-    SvGROW(TARG, len + 1);
-
-    Copy(start, SvPVX(TARG), len, U8);
-    SvPVX(TARG)[len] = '\0';
-    SvPOK_only_UTF8(TARG);
-    SvCUR_set(TARG, len);
+    sv_setpvn(TARG, (const char *)start, len);
 
     if (DO_UTF8(source))
         SvUTF8_on(TARG);
     else
         SvUTF8_off(TARG);
 
-    if (SvTAINTED(source))
-        SvTAINT(TARG);
-
-    SvSETMAGIC(TARG);
-
-    SETs(TARG);
+    SETTARG;
 
     XSRETURN(1);
 }