Implement CORE::<lang-rev> pseudo-packages

Extend `CORE` with second-level packages representing individual cores
for specific language revisions. I.e, `CORE::C` is for `CORE.setting`
or, correspondingly, for 6.c language. `CORE::D`, `CORE::E`, and so on.
New letters become available as soon as a `CORE` context with
`CORE-SETTING-REV` symbol becomes available.

Each subpackage has `PRECISE_SCOPE`. Thus, `CORE::D::Any` will fail
because `Any` is not defined in `CORE.d.setting`.

Raku/problem-solving#80

src/core.d/core_prologue.pm6

@@ -1,6 +1,6 @@
 use nqp;
 
-# This constant is only to support tests.
+# This constant must specify current CORE revision
 my constant CORE-SETTING-REV = 'd';
 
 # vim: ft=perl6 expandtab sw=4

src/core.e/PseudoStash.pm6

@@ -43,7 +43,7 @@ my class PseudoStash is Map {
                 nqp::bindattr($stash, Map, '$!storage', nqp::ctxlexpad($ctx)),
                 nqp::bindattr($stash, PseudoStash, '$!ctx', $ctx),
                 nqp::bindattr_i($stash, PseudoStash, '$!mode', STATIC_CHAIN),
-                $stash.pseudo-package('CORE')
+                $stash.pseudo-package('CORE'),
               )
             )
         },
@@ -187,10 +187,46 @@ my class PseudoStash is Map {
         }
     );
 
+
+    method !find-rev-core($key) {
+        my $rev = nqp::lc($key);
+        my $ctx := $!ctx;
+        my $found := nqp::null();
+        my $stash;
+        nqp::while(
+            $ctx && !$found,
+            nqp::stmts(
+                (my $lexpad := nqp::ctxlexpad($ctx)),
+                nqp::if(
+                    nqp::existskey($lexpad, 'CORE-SETTING-REV')
+                    && nqp::iseq_s($rev, nqp::atkey($lexpad, 'CORE-SETTING-REV')),
+                    ($found := $ctx),
+                    ($ctx := nqp::ctxouterskipthunks($ctx))
+                ),
+            )
+        );
+        nqp::if(
+            nqp::isnull($found),
+            Failure.new(X::NoCoreRevision.new(lang-rev => $rev)),
+            nqp::stmts(
+                ($stash := nqp::create(PseudoStash)),
+                nqp::bindattr($stash, Map, '$!storage', nqp::ctxlexpad($found)),
+                nqp::bindattr($stash, PseudoStash, '$!ctx', $found),
+                nqp::bindattr_i($stash, PseudoStash, '$!mode', PRECISE_SCOPE),
+                $stash.pseudo-package($key)
+            )
+        )
+    }
+
     multi method AT-KEY(PseudoStash:D: Str() $key) is raw {
-        my Mu $val := nqp::null();
+        my $name := $!package.^name;
+        my Mu $val := nqp::if(
+            (nqp::iseq_s($name, 'CORE') && nqp::iseq_i(nqp::chars($key), 1)),
+            self!find-rev-core($key),
+            nqp::null()
+        );
         nqp::if(
-          nqp::existskey($pseudoers,$key),
+          nqp::isnull($val) && nqp::existskey($pseudoers,$key),
           ($val := nqp::atkey($pseudoers,$key)(self)),
           nqp::stmts(
             nqp::if(          # PRECISE_SCOPE is exclusive

src/core.e/core_prologue.pm6

@@ -1,6 +1,6 @@
 use nqp;
 
-# This constant is only to support tests.
+# This constant must specify current CORE revision
 my constant CORE-SETTING-REV = 'e';
 
 # vim: ft=perl6 expandtab sw=4

src/core/Exception.pm6

@@ -2603,6 +2603,11 @@ my class X::NoSuchSymbol is Exception {
     method message { "No such symbol '$.symbol'" }
 }
 
+my class X::NoCoreRevision is Exception {
+    has $.lang-rev;
+    method message { "No CORE for language version 6.$!lang-rev" }
+}
+
 my class X::Item is Exception {
     has $.aggregate;
     has $.index;

src/core/core_prologue.pm6

@@ -68,7 +68,7 @@ PROCESS::<$SCHEDULER> = JavaScriptScheduler.new();
 BEGIN {nqp::p6setassociativetype(Associative);}
 #?endif
 
-# This constant is only to support tests.
+# This constant must specify current CORE revision
 my constant CORE-SETTING-REV = 'c';
 
 # vim: ft=perl6 expandtab sw=4