Issue 12421 - Allow simpler syntax for lambda template declarations

https://issues.dlang.org/show_bug.cgi?id=12421
Well this is problematic but also useful, [Issue 5503 – Forward declarations for nested mutually recursive functions](https://issues.dlang.org/show_bug.cgi?id=5503).
Enforcing lexical scoping here would mean that we'd have to snapshot (set a mark) all symbols when the template is declared and later restrict access to those symbols. I'm not sure it's worth too much trouble. Would it be possible to simply push a new scope after the lambda?