YCP Killer: Add |result| attribute to |Call| nodes

ycpc now provides information whether a call result was used or not. We
include it in the YCP AST so that it can be used to simplify calls with
references (this will come in the next commit).
YCP AST: Simplify compilation of calls with reference arguments

There is no need to compile calls with reference arguments whose result
is unused as |Ruby::Expression|. They can be compiled as
|Ruby::Statements| and the variable storing the result can be omitted.
This leads to a bit nicer code.