Support new macro syntax

[Folyd]

Rust has a new macro syntax, for example std::pin::pin.

pub macro pin($value:expr $(,)?) {
    ...
}

Here is the error log for this pin!() macro:

Uncaught (in promise) macro_railroad parse failed: expected `macro_rules`

[lukaslueg]

declarative macros 2.0 are not yet stable (rust-lang/rust#39412), although stdlib gets to use them (as it does with all unstable features). Let's wait for 39412 to settle