support `where` syntax in constructors
- use it in a few places in Base as a demo and test
- allow `x where (T,S)` syntax, parsed as `(where x T S)`
- use `where` in generated default constructors
- in constructor defs `Foo{A,B}(...) where ... = new(...)`, copy parameters from
`Foo` to `new`
- fix error message for `x where t = y`
