Fix deprecations of $*OS/$*OSVER

Basically:
- expose Obsolete as a proper class
- make sure there are 'say' candidates for Obsolete:D: that do .gist

src/core/Deprecations.pm

@@ -1,6 +1,17 @@
 
 my %DEPRECATIONS; # where we keep our deprecation info
 
+class Obsolete is Str {
+    has $!name;
+    has $!value;
+    has $!instead;
+
+    submethod BUILD (:$!name, :$!value, :$!instead) {}
+
+    method Str  { DEPRECATED( $!instead, :up(2), :what($!name) ); $!value }
+    method gist { DEPRECATED( $!instead, :up(2), :what($!name) ); $!value }
+}
+
 class Deprecation {
     has $.file;         # file of the code that is deprecated
     has $.type;         # type of code (sub/method etc.) that is deprecated
@@ -39,17 +50,6 @@ class Deprecation {
         $message;
     }
 
-    class Obsolete is Str {
-        has $!name;
-        has $!value;
-        has $!instead;
-
-        submethod BUILD (:$!name, :$!value, :$!instead) {}
-
-        method Str  { DEPRECATED( $!instead, :up(3), :what($!name) ); $!value }
-        method gist { DEPRECATED( $!instead, :up(3), :what($!name) ); $!value }
-    }
-
     # system variable deprecations
     method obsolete (|c) { Obsolete.new(|c) }
 }

src/core/IO.pm

@@ -8,6 +8,11 @@ sub print(|) {
 }
 
 proto sub say(|) { * }
+multi sub say(Obsolete:D \x) {
+    my $out := $*OUT;
+    $out.print(x.gist);
+    $out.print("\n");
+}
 multi sub say(Str:D \x) {
     my $out := $*OUT;
     $out.print(x);

src/core/Mu.pm

@@ -1,6 +1,7 @@
 my class X::Constructor::Positional { ... }
 my class X::Method::NotFound        { ... }
 my class X::Method::InvalidQualifier { ... }
+my class Obsolete { ... }
 
 my class Mu { # declared in BOOTSTRAP
     proto method ACCEPTS(|) { * }
@@ -413,6 +414,7 @@ my class Mu { # declared in BOOTSTRAP
     method item(Mu \item:) is rw { item }
 
     proto method say(|) { * }
+    multi method say(Obsolete:D:) { say(self.gist) }
     multi method say() { say(self) }
     method print() { print(self) }