syntax: allow `trace_macros!` and `log_syntax!` in item position.

Previously

    trace_macros!(true)
    fn main() {}

would complain about `trace_macros` being an expression macro in item
position. This is a pointless limitation, because the macro is purely
compile-time, with no runtime effect. (And similarly for log_syntax.)

This also changes the behaviour of `trace_macros!` very slightly, it
used to be equivalent to

    macro_rules! trace_macros {
        (true $($_x: tt)*) => { true };
        (false $($_x: tt)*) => { false }
    }

I.e. you could invoke it with arbitrary trailing arguments, which were
ignored. It is changed to accept only exactly `true` or `false` (with no
trailing arguments) and expands to `()`.
auto merge of #12948 : olleolleolle/rust/master, r=huonw

Rust doc sprint: adding doc strings to the Terminfo library.

This is my very first Rust repository PR, so please do not hold back any formatting, nit-picky commentary. I need it.
auto merge of #12998 : huonw/rust/log_syntax, r=alexcrichton

syntax: allow `trace_macros!` and `log_syntax!` in item position.

Previously

    trace_macros!(true)
    fn main() {}

would complain about `trace_macros` being an expression macro in item
position. This is a pointless limitation, because the macro is purely
compile-time, with no runtime effect. (And similarly for log_syntax.)

This also changes the behaviour of `trace_macros!` very slightly, it
used to be equivalent to

    macro_rules! trace_macros {
        (true $($_x: tt)*) => { true };
        (false $($_x: tt)*) => { false }
    }

I.e. you could invoke it with arbitrary trailing arguments, which were
ignored. It is changed to accept only exactly `true` or `false` (with no
trailing arguments) and expands to `()`.