Bump Rust toolchain version for CI to 1.64

[bdbai]

This aims to unblock #12 .

Since core::ptr::addr_of is not stablized until Rust 1.51, the current toolchain version used in appveyor is too old to build. According to the MSVC policy for this project i.e. Always the latest stable, we can update the specified version to let CI pass.

[poliorcetics]

Raising the MSRV to 1.51 should be enough, even if it's possible to raise to 1.64 there is no need to. It would be less disruptive across the ecosystem too

[bdbai]

@poliorcetics unfortunately the current nightly toolchain (that some CI checks are using) already rejects the code 😭 That means I have to do the upgrade and fix in one go. I have rebased #12 and this might have to be closed.

I believe the version specified here is used for CI checks only and probably will not affect downstream users.