Give QuantHash/Capture a proper .item method

Fixes #1862.  Kudos to Ralph Mellor for mentioning this.

Following jnthn's comment then:

    If  something decides that it's going to implement .iterator
    without making itself Iterable, then it's on that type to also
    implement method item,  I'd say

src/core.c/Capture.pm6

@@ -4,6 +4,7 @@ my class Capture { # declared in BOOTSTRAP
     #     has %!hash;   # named parameters
 
     method from-args(|c) { c }
+    method item() is raw { my $ = self }
 
     submethod BUILD(:@list, :%hash --> Nil) {
         my Int:D $elems = @list.elems; # force reification of all

src/core.c/QuantHash.pm6

@@ -1,6 +1,7 @@
 my role QuantHash does Associative {
 
     method keyof() { Mu }
+    method item() is raw { my $ = self }
 
     method SET-SELF(QuantHash:D: \elems) is implementation-detail {
         nqp::bindattr(self,::?CLASS,'$!elems',elems)