Remove casts to size_t in Rust code

[briansmith]

See rust-lang/rust/#28096.

There are some casts like digest.len() as libc::size_t that are potentially truncating and thus unsafe. At the same time, the comments in the code questioning the safety of such casts need to be removed.

This will have to wait for a higher-versioned released of the libc crate with rust-lang/rust#28096 merged into it.

[briansmith]

This depends on rust-lang/rust#28779. That was merged to the Rust beta branch in rust-lang/rust#29112 so we can probably do this soon.

[briansmith]

Fixed in 377f611 by dropping the dependency on the libc crate and defining our own size_t type: ring::c::size_t.