Remove "full" feature from syn

[Pratyush]

It seems that the library might not need to use the full feature of syn. This PR removes it to potentially help improve compile times for end users. If I'm missing something please let me know =).

[mcarton]

Thanks!

[kankri]

This seems to have broken #derivative(Default(value="expr")) where the expression is a tuple, struct, list, method call, etc.... For example, this compiles OK with commit 1fd45f3 but fails after the change in this PR:

pub struct Bar { x: u32, y: u32 }

#[derive(Derivative)]
#[derivative(Default)]
pub struct Foo {
	#[derivative(Default(value="Bar {x:42, y:42}"))]
	a: Bar,
	#[derivative(Default(value="[42;2]"))]
	b: [u32; 2],
}

Using commit fbbd067 you get either

error: proc-macro derive panicked
help: message: unexpected token

or

error: proc-macro derive panicked
help: message: unsupported expression; enable syn's features=["full"]

[dtolnay]

Looks like

rust-derivative/src/attr.rs

Line 370 in 0bd0e52

out.default.value = Some(try!(parse_str(&value)));

should be parsing to a TokenStream rather than Expr.

[mcarton]

Looks like

rust-derivative/src/attr.rs

Line 370 in 0bd0e52

out.default.value = Some(try!(parse_str(&value)));

should be parsing to a TokenStream rather than Expr.

That did the trick, thanks!