Tutorial: Update "Interacting with foreign code" to not use "uint"

[jruderman]

The example at the top of section 16 "Interacting with foreign code" warns:

/Users/jruderman/Desktop/7.rs:6:26: 6:30 warning: found rust type `uint` in foreign module, while libc::c_uint or libc::c_ulong should be used
/Users/jruderman/Desktop/7.rs:6     fn SHA1(src: *u8, sz: uint, out: *u8) -> *u8;
                                                          ^~~~

Sections 16.3 "Unsafe pointers" and 16.6 "Passing structures" also need to be updated to not use uint.

[catamorphism]

On it.