Make bindgen use `usize` for `size_t`.

Technically, size_t could be something other than the pointer width of the architecture, but for all architectures that Rust supports or is likely to support in the future, they will be the same.

This flag saves having to write as usize a bunch of times, without losing anything. It also makes it easier in the future to switch this auto conversion off in the future, if that becomes necessary.