Better syntax checking on attributes

[kmcallister]

For example, #![crate_type(lib)] produces

error: main function not found
error: aborting due to previous error

which is not a very good clue to the correct syntax #![crate_type="lib"].

[steveklabnik]

This example now shows

hello.rs:1:1: 1:20 error: `crate_type` requires a value
hello.rs:1 #![crate_type(lib)]
           ^~~~~~~~~~~~~~~~~~~
note: for example: `#![crate_type="lib"]`
error: aborting due to previous error

Showing off the exact syntax.