[perl #126341] STable conflict with .^add_method

[Tokuhiro Matsuno]

# New Ticket Created by  "Tokuhiro Matsuno" 
# Please include the string:  [perl #126341]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=126341 >


Here is a code that throws exception.
```
#!/bin/bash

mkdir lib/

cat > lib/Child.pm6 <<EOF;
use v6;
use Parent;

unit class Child is Parent;
EOF

cat > lib/Parent.pm6 <<EOF;
use v6;
unit class Parent;

::?CLASS.^add_method: 'x', method () { };
EOF

perl6-m -Ilib --target=mbc --output=lib/Parent.pm6.moarvm lib/Parent.pm6
perl6-m -Ilib --target=mbc --output=lib/Child.pm6.moarvm lib/Child.pm6

perl6-m -Ilib -e 'use Child'
```

Output is:
```
===SORRY!===
STable conflict detected during deserialization.
(Probable attempt to load two modules that cannot be loaded together).
```

I guess this code should work(I know it works if i enclose MOP methods by BEGIN 
block).
If it isn't, perl6-m should show more human readable message.