Simple demonstration of macro non-hygiene in Rust

This demonstrates a couple places where hygiene is lacking with macros as implemented currently in Rust.

• Type references use whatever name is in scope:

  The thing!() macro builds a Thing, expecting it to have the type in scope, and be the correct type. The line marked 'good1' in main.rs is necessary to be able to have this invocation work in the main program. If, this is removed, and replaced with bad1, the macro expansion will fail. The macro will reference the locally defined type instead of the one defined in the scope of the macro, and will fail because the field name is incorrect.

• Function names have similar problems, as can be seen by the blop function and the see_blop!() macro that expands to a call to it.

Using a full path to either the type or function name doesn't work, because there is no consistent path that can refer to this function. For example, in the library, ::stuff::Thing would work, but in another module, it would likely need to be macros::stuff::Thing. Even then, the name could change because of an extern crate macros as ... declaration.