Make X::OS "is Exception".

It now passes it on to the various classes that do it, much as we do
with X::Comp. This in turn means using X::OS and X::IO directly will
give something that type matches Exception.

src/core/Exception.pm

@@ -253,45 +253,45 @@ do {
 
 }
 
-my role X::OS {
+my role X::OS is Exception {
     has $.os-error;
 }
 
 my role X::IO does X::OS { };
 
-my class X::IO::Rename does X::IO is Exception {
+my class X::IO::Rename does X::IO {
     has $.from;
     has $.to;
     method message() {
         "Failed to rename '$.from' to '$.to': $.os-error"
     }
 }
 
-my class X::IO::Copy does X::IO is Exception {
+my class X::IO::Copy does X::IO {
     has $.from;
     has $.to;
     method message() {
         "Failed to copy '$.from' to '$.to': $.os-error"
     }
 }
 
-my class X::IO::DoesNotExist does X::IO is Exception {
+my class X::IO::DoesNotExist does X::IO {
     has $.path;
     has $.trying;
     method message() {
         "Failed to find '$.path' while trying to do '.$.trying'"
     }
 }
 
-my class X::IO::NotAFile does X::IO is Exception {
+my class X::IO::NotAFile does X::IO {
     has $.path;
     has $.trying;
     method message() {
         "'$.path' is not a regular file while trying to do '.$.trying'"
     }
 }
 
-my class X::IO::Directory does X::IO is Exception {
+my class X::IO::Directory does X::IO {
     has $.path;
     has $.trying;
     has $.use;
@@ -302,65 +302,65 @@ my class X::IO::Directory does X::IO is Exception {
     }
 }
 
-my class X::IO::Symlink does X::IO is Exception {
+my class X::IO::Symlink does X::IO {
     has $.target;
     has $.name;
     method message() {
         "Failed to create symlink called '$.name' on target '$.target': $.os-error"
     }
 }
 
-my class X::IO::Link does X::IO is Exception {
+my class X::IO::Link does X::IO {
     has $.target;
     has $.name;
     method message() {
         "Failed to create link called '$.name' on target '$.target': $.os-error"
     }
 }
 
-my class X::IO::Mkdir does X::IO is Exception {
+my class X::IO::Mkdir does X::IO {
     has $.path;
     has $.mode;
     method message() {
         "Failed to create directory '$.path' with mode '0o{$.mode.fmt("%03o")}': $.os-error"
     }
 }
 
-my class X::IO::Chdir does X::IO is Exception {
+my class X::IO::Chdir does X::IO {
     has $.path;
     method message() {
         "Failed to change the working directory to '$.path': $.os-error"
     }
 }
 
-my class X::IO::Dir does X::IO is Exception {
+my class X::IO::Dir does X::IO {
     has $.path;
     method message() {
         "Failed to get the directory contents of '$.path': $.os-error"
     }
 }
 
-my class X::IO::Cwd does X::IO is Exception {
+my class X::IO::Cwd does X::IO {
     method message() {
         "Failed to get the working directory: $.os-error"
     }
 }
 
-my class X::IO::Rmdir does X::IO is Exception {
+my class X::IO::Rmdir does X::IO {
     has $.path;
     method message() {
         "Failed to remove the directory '$.path': $.os-error"
     }
 }
 
-my class X::IO::Unlink does X::IO is Exception {
+my class X::IO::Unlink does X::IO {
     has $.path;
     method message() {
         "Failed to remove the file '$.path': $.os-error"
     }
 }
 
-my class X::IO::Chmod does X::IO is Exception {
+my class X::IO::Chmod does X::IO {
     has $.path;
     has $.mode;
     method message() {