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

[huonw]

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 ().

[alexcrichton]

r=me, but a test may need to be fixed on travis