interpolate_idents

You cannot currently define a struct, enum, function, or field using concat_idents! due to the way macros are parsed by the Rust compiler. This will hopefully change in the future, but interpolate_idents! sloppily solves a side effect of the currently lacking macro system today.

```rust

![feature(plugin)]

![plugin(interpolate_idents)]

macrorules! makefn { ($x:ident) => ( interpolateidents! { fn my $x _fn -> u32 { 1000 } } ) } ```

Now make_fn!(favorite); is equivalent to fn my_favorite_fn() -> u32 { 1000 }.

In short, surround multiple space-separated identifiers (or macro identifer variables) with square brackets to concatenate the identifiers. Check tests/tests.rs for another example.

This plugin was quickly hacked together. It is likely not performant and most certainly not readable.