[perl #132146] [LTA] Consistency of `&` coercer

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# New Ticket Created by  Zoffix Znet 
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IRC: https://irclog.perlgeek.de/perl6/2017-09-22#i_15205100

If you give the `&` coercer some stuff that isn't a callable it just gives you 
the stuff back.
However, if the "stuff" you give is a sigil-less constant, it gives a Nil. Yet, 
if you parenthesize
that constant, you do get its actual value:

    22:13       Zoffix  m: constant $z = 42; say &$z
    22:13       camelia rakudo-moar ffd179: OUTPUT: «42␤»
    22:14       Zoffix  m: constant z = 42; say &z
    22:14       camelia rakudo-moar ffd179: OUTPUT: «Nil␤»
    22:14       Zoffix  inconsistent
    22:14               m: constant z = 42; say &(z)
    22:14       camelia rakudo-moar ffd179: OUTPUT: «42␤»

Feels like the Nil version needs to either return the actual value or throw; 
not silently return the wrong value.