- added 'nil' keyword to lang doc
(did not add 'true' and 'false' because I think these don't need to be
keywords, they can be recreated from whole cloth)
- added 'new' syntax to expressions
(this is needed because of the type as 1st argument - the expression
list is probably not yet implemented, but I think we may want it
eventually)
- removed optional ';'s in sieve example - make it more concise
- fixed several bugs in go.atg, no parses sieve
- added sieve as test case