[perl #123673] Rakudo silently loses information on .encode for non-representable characters

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# New Ticket Created by  Moritz Lenz 
# Please include the string:  [perl #123673]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=123673 >


<moritz> m: say 'ö'.encode('ASCII')
<camelia> rakudo-moar 7e8d8a: OUTPUT«Blob[uint8]:0x<3f>␤»
<moritz> that looks like a bug to me

Since ö isn't representable as ASCII, this should throw an exception.
Currently it encodes to 0x37, which is the question mark / replacement
character. Since using a replacmement character silently loses 
information, which is a dangerous default.

Here is my wish list:

1) the default is to throw an exception, let's say

     X::Str::NotEncodable.new(
         source => 'ö',
         index  => '0',
         destination => 'ASII',
     );

2) if a replacment is desired, indicate that through an adverb in the
.encode call, either

     'ö'.encode('ASCII', :replacement)

to get the default, or

     'ö'.encode('ASCII'. :replacement(Buf.new(42)))

to be able to chose a replacement byte or byte sequence.

If somebody implements it, I'll add it to the design docs :-)

Cheers,
Moritz