Log Message:
p5-Data-Alias: update to 1.21.

version 1.21; 2017-12-08

  * update for context stack changes in Perl 5.23.8

  * update to accommodate PERL_OP_PARENT builds of Perl 5.21.11 or later
    (which is the default from Perl 5.25.1)

  * update for removal of LEX_KNOWNEXT in Perl 5.25.1

  * update test suite to not rely on . in @INC, which is no longer
    necessarily there from Perl 5.25.7

  * use cleaner wrap_op_checker() API to control op checking