[perl #130843] [MOAR][IO] .seek(... SeekFromCurrent) seeks to incorrect place if called after .readchars

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# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #130843]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=130843 >


If .seek(... SeekFromCurrent) is called after a call to .readchars, the 
position sought to appears to be way off target:

    22:54       IOninja m: "/tmp/Foo.pm6".IO.spurt: "I love you so very much"; 
with "/tmp/Foo.pm6".IO.open { .read(2); .seek: 1, SeekFromCurrent; .tell.say }
    22:54       camelia rakudo-moar 5ec251: OUTPUT: «3␤»
    22:55       IOninja m: "/tmp/Foo.pm6".IO.spurt: "I love you so very much"; 
with "/tmp/Foo.pm6".IO.open { .readchars(2); .seek: 1, SeekFromCurrent; 
.tell.say }
    22:55       camelia rakudo-moar 5ec251: OUTPUT: «24␤»
    22:55       IOninja looks .readchars messes up the current position as far 
as seekage is concerned (it's fine if you try to read more, for example)

After some discussion on IRC (https://irclog.perlgeek.de/perl6-dev/2017-02-22 
), one suggestion was that there's some sort of buffering involved:

    23:00               m: "/tmp/Foo.pm6".IO.spurt: "I love you\nso very much"; 
with "/tmp/Foo.pm6".IO.open { .readchars(2); .seek: 1, SeekFromCurrent; 
.tell.say }
    23:00       camelia rakudo-moar 5ec251: OUTPUT: «24␤»
    23:00       IOninja m: "/tmp/Foo.pm6".IO.spurt: "I love you\nso very much" 
x 100; with "/tmp/Foo.pm6".IO.open { .readchars(2); .seek: 1, SeekFromCurrent; 
.tell.say }
    23:00       camelia rakudo-moar 5ec251: OUTPUT: «2301␤»
    23:00       jnthn   Anyway, just sayin' that if it's off-by-one it's 
actually not lying :)
    23:00       IOninja m: "/tmp/Foo.pm6".IO.spurt: "I love you\nso very much" 
x 10000; with "/tmp/Foo.pm6".IO.open { .readchars(2); .seek: 1, 
SeekFromCurrent; .tell.say }
    23:00       camelia rakudo-moar 5ec251: OUTPUT: «32769␤»
    23:00       IOninja m: "/tmp/Foo.pm6".IO.spurt: "I love you\nso very much" 
x 20000; with "/tmp/Foo.pm6".IO.open { .readchars(2); .seek: 1, 
SeekFromCurrent; .tell.say }
    23:00       camelia rakudo-moar 5ec251: OUTPUT: «32769␤»

This behaviour is not present on JVM.