Make varargs closures variable argument collectors

The arguments partial application using insertArguments sometimes drops
the variable argument collector status of method handles. This raises
bugs in, say, dynamic object method dispatch.

This fixes #79 by generating some extra bytecode to call
asVarargsCollector after the partial application.
Spurious DUP
Reproduce the original test case for #79
Deal with varargs closures with more than 1 parameter
Contrieved test case
Merge pull request #80 from jponge/wip/fix-79

Make varargs closures variable argument collectors