Macro delog::generate_macros

macro_rules! generate_macros {
    () => { ... };
}

Generate logging macros that can be gated by library.

Realize that these macros are generated in the namespace of the consuming library, the one that actally later makes calls to local_warn! etc.

To see this in action, compile documentation using cargo doc --features example, or inspect the gate-tests/ subdirectory.