fix Y combinator to adapt to changing role name -> Definition
represent Perl6 Terms by TTerms (fn objects)
fix MethodFixedPoint and add tests (now definitely works with things represented as lambdaFn)
make AppT and LamT roles, too; hide AlphaVarNode
begin replacing Substitution functions with functional (lambda) implementations
git-ignore file _prove
add Streams (lazy TList s), add helper fn if-nil for reducing nr of calls to a list instance fn by half