Moved to the latest rascal and rascal-plugin release

[DavyLandman]

@jurgenvinju & @PaulKlint I think rascal-core is finding a few errors that might not be errors:

• https://github.com/usethesource/rascal/issues/1956
• the nothing() of util::Maybe in a kwparam default is causing the following error:

  [ERROR]   lang/flybytes/Compiler.rsc:032:049: Default expression of type `Maybe[loc]` expected, found `Maybe[&A]`

apart from that, for @jurgenvinju to pickup, there are quite some errors where the overloads have different return types. In this PR I fixed a few of the type errors that where easy to fix, but these I'll leave for you.

[jurgenvinju]

I fixed those return type things last week on a local branch. So I'll merge that a.s.a.p.

[DavyLandman]

@jurgenvinju & @PaulKlint the code now typechecks, there are now 2 errors in the tests.

[DavyLandman]

@jurgenvinju this one is ready for merging I think.