Proposal: forward compatibility

[Jason House]

D is an evolving language that does not restrict itself by requiring backwards compatibility. Instead I propose that D adopt forward compatibility... 

Specifically, allow D1 code to accept D2 code without doing extra functionality. In D1, his would allow defining constants to be declared with either const or invariant, object invariants to allow parenthesis,  and ignoring transitive const elements of type declarations.

The goal is to not change the behavior of existing D1 code, but to accept previously invalid D1 code. This should make the job of maintaining a dual code base much easier.