Compiler Plugins example in Rust Book does not compile

[rprichard]

Compiling the current version of roman_numerals.rs produces:

$ rustc roman_numerals.rs 
roman_numerals.rs:37:24: 37:49 error: mismatched types:
 expected `&collections::string::String`,
    found `&str`
(expected struct `collections::string::String`,
    found str) [E0308]
roman_numerals.rs:37                 text = text.slice_from(rn.len());
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~
roman_numerals.rs:46:38: 46:43 error: mismatched types:
 expected `usize`,
    found `u32`
(expected usize,
    found u32) [E0308]
roman_numerals.rs:46     MacEager::expr(cx.expr_usize(sp, total))
                                                          ^~~~~
error: aborting due to 2 previous errors

I'll submit a PR with a fix once I've run make check on it...