Pin zerocopy to 0.3.0

[WesleyRosenblum]

zerocopy 0.3.1 does not build in Stable rust

error[E0658]: const generics are unstable
    --> /usr/local/cargo/registry/src/git.luolix.top-1ecc6299db9ec823/zerocopy-0.3.1/src/lib.rs:71:38
     |
 71  |         unsafe impl<T: $trait, const N: usize> $trait for [T; N] {
     |                                      ^
 ...
 263 | impl_for_composite_types!(FromBytes);
     | ------------------------------------- in this macro invocation
     |
     = note: see issue #74878 <https://github.com/rust-lang/rust/issues/74878> for more information
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

This PR (openrr/openrr#216) has tagged the zerocopy author, we can see if he responds on whether this was intentional

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[WesleyRosenblum]

Looks like this feature will be stable on March 25: rust-lang/rust#79135, but I guess that Rust version (1.51) is going to be too new for us though