Make Str.trim-leading between 20x and 90x faster

- about 20x for strings without whitespace at start
- about 90x for strings *with* whitespace at start

Inspired by the notion that nqp::substr is actually optimised to
return the source string if the entire string is indicated.  So we
don't need to do anything special in the setting.

src/core.c/Str.pm6

@@ -2200,11 +2200,12 @@ my class Str does Stringy { # declared in BOOTSTRAP
     }
 
     method trim-leading(Str:D: --> Str:D) {
-        my str $str = nqp::unbox_s(self);
-        my int $pos = nqp::findnotcclass(
-                          nqp::const::CCLASS_WHITESPACE,
-                          $str, 0, nqp::chars($str));
-        $pos ?? nqp::p6box_s(nqp::substr($str, $pos)) !! self;
+        nqp::substr(
+          self,
+          nqp::findnotcclass(
+            nqp::const::CCLASS_WHITESPACE,self,0,nqp::chars(self)
+          )
+        )
     }
 
     method trim-trailing(Str:D: --> Str:D) {