pugs, javascript backend, and perl5

[Chia-liang Kao]

Hi,

I've checked-in some changes for pugs' javascript backend to support
Perl5, which means we can now implement io primitives with perl5 via
javascript, as well as supporting eval($code, :lang<perl5>) and
more transparent CPAN module reusing.

I've implemented slurp() as a starting point:

$ ./runjs.pl --run=jspm -e 'say slurp("README").bytes'
Perl5 required.

$ ./runjs.pl --perl5 --run=jspm -e 'say slurp("README").bytes'
19981

Cheers,
CLK