Remove caching of Seqs

This is a temporary partial commit. If you see it on master then
things went very wrong.

src/core.c/Seq.pm6

@@ -5,6 +5,9 @@ my class Seq is Cool does Iterable does Sequence {
     # way through. Can only be obtained once.
     has Iterator $!iter;
 
+    # The number of values that have been produced
+    has int $!produced;
+
     # The only valid way to create a Seq directly is by giving it the
     # iterator it will consume and maybe memoize.
     multi method new(Seq: Iterator:D $iter) {
@@ -41,6 +44,33 @@ my class Seq is Cool does Iterable does Sequence {
         )
     }
 
+    proto method AT-POS(|) {*}
+    multi method AT-POS(Seq:D: Int:D $idx) is raw {
+        if $idx < $!produced {
+            die "asked $idx, which was already produced";
+        }
+        else {
+            my int $skip = $idx - $!produced;
+            $!produced = $idx;
+            $!iter.skip($skip)
+              ?? $!iter.pull-one
+              !! Nil
+        }
+    }
+
+    multi method AT-POS(Seq:D: int $idx) is raw {
+        if $idx < $!produced {
+            die "asked $idx, which was already produced";
+        }
+        else {
+            my int $skip = $idx - $!produced;
+            $!produced = $idx;
+            $!iter.skip($skip)
+              ?? $!iter.pull-one
+              !! Nil
+        }
+    }
+
     multi method Seq(Seq:D:)   { self }
 
     method Capture() {

src/core.c/Sequence.pm6

@@ -57,13 +57,13 @@ my role Sequence does PositionalBindFailover {
 
     method Numeric(::?CLASS:D:) { self.cache.elems }
 
-    multi method AT-POS(::?CLASS:D: Int:D $idx) is raw {
-        self.cache.AT-POS($idx)
-    }
+    #multi method AT-POS(::?CLASS:D: Int:D $idx) is raw {
+    #    self.cache.AT-POS($idx)
+    #}
 
-    multi method AT-POS(::?CLASS:D: int $idx) is raw {
-        self.cache.AT-POS($idx)
-    }
+    #multi method AT-POS(::?CLASS:D: int $idx) is raw {
+    #    self.cache.AT-POS($idx)
+    #}
 
     multi method EXISTS-POS(::?CLASS:D: Int:D $idx) {
         self.cache.EXISTS-POS($idx)