[perl #130840] [JVM] IO::Handle.readchars incorrectly assumes all chars are 2 bytes long

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# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #130840]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=130840 >


Based on the code, seems it's a naive kludge for the unimplemented 
nqp::readcharsfh op. As a result, .readchars on JVM reads incorrect number
of chars, at times producing wrong data altogether:

<ZoffixW> j: "/tmp/Foo.pm6".IO.spurt: "I♥u so very much"; 
"/tmp/Foo.pm6".IO.open.readchars(3).say
<camelia> rakudo-jvm fb4f16: OUTPUT: «I♥u ␤»
<ZoffixW> j: "/tmp/Foo.pm6".IO.spurt: "I love you so very much"; 
"/tmp/Foo.pm6".IO.open.readchars(3).say
<camelia> rakudo-jvm fb4f16: OUTPUT: «I love␤»
<ZoffixW> j: "/tmp/Foo.pm6".IO.spurt: "♥"; 
"/tmp/Foo.pm6".IO.open.readchars(1).say
<camelia> rakudo-jvm fb4f16: OUTPUT: «�␤»