VIP: enforce defaults to be ... in interfaces

[charles-cooper]

          > btw, it should be allowed to use `= ...` for defaults, so you don't have to specify a specific value (should the value be enforced if present?)

That's a good point. Maybe we should always enforce defaults to be ... in interfaces

Originally posted by @charles-cooper in #3170 (comment)

per @fubuloubu #3170 (comment)

[cyberthirst]

i'm in favor of enforcing this

1. simplifies the implements statement (although this would loose some comp time guarantees of what the default value is - but we're not enforcing it now anyway)

2. easier for users to understand. if a concrete value is provided it's fairly easy to assume that it's filled at callsite - while the semantics are "interface with a contract which implements a default value for this parameter"