Macro mentat::kw

macro_rules! kw {
    ( : $n:ident ) => { ... };
    ( : $ns:ident / $n:ident ) => { ... };
    ( : $ns:ident$(. $nss:ident)+ / $n:ident ) => { ... };
}

Produce the appropriate Keyword for the provided namespace and name. This lives here because we can't re-export macros: https://github.com/rust-lang/rust/issues/29638.