Improve conventions for passing fixed-sized arrays to FFI functions

[briansmith]

After I filed rust-lang/rust#30382 the feedback was mostly negative so I made it the convention to use x: *const T/*[N]*/ and x: *mut T/*[N]*/ instead of x: &[T; N] and x: &mut [T; N] in FFI function declarations. However, now we have rust-lang/rust#31227 and it seems there's pretty widespread agreement that the latter forms are better, so we should just use them everywhere applicable.

See also #129.