Update azure to use latest, fix warnings

[yeastplume]

Further update to CI settings and fix for warnings:

• Use ubuntu-latest, windows-latest and macos-latest images
• Fix dyn warning
• Modify all instances of mem::uninitialized() to mem::MaybeUninit::uninit().assume_init()

[quentinlesceller]

Since rustup is installed you need to add:

macOS and Linux:

rustup toolchain install $RUSTUP_TOOLCHAIN
rustup default $RUSTUP_TOOLCHAIN

and Windows:

rustup toolchain install %RUSTUP_TOOLCHAIN%
rustup default %RUSTUP_TOOLCHAIN%

[quentinlesceller]

It looks like Rustup is not included in Windows Server 2019 (https://github.com/actions/virtual-environments/blob/master/images/win/Windows2019-Readme.md). You might want to try the above. Otherwise you need to add:

    - script: |
         curl -sSf -o rustup-init.exe https://win.rustup.rs
         rustup-init.exe -y --default-toolchain %RUSTUP_TOOLCHAIN%
       displayName: Install Rust

[yeastplume]

Unfortunately on windows that brings us right back to the original issue:

https://dev.azure.com/mimblewimble/rust-secp256k1-zkp/_build/results?buildId=1653&view=logs&j=57b2b475-8d2a-5b04-5d48-0e52fa3a6ad3&t=5ecf445a-2f64-5417-8d77-bb5b450b5bee

[quentinlesceller]

@yeastplume can you try this on Windows:

rustup-init.exe toolchain install  %RUSTUP_TOOLCHAIN%
rustup-init.exe default %RUSTUP_TOOLCHAIN%

and remove this line:

rustup-init.exe -y --default-toolchain %RUSTUP_TOOLCHAIN%

[quentinlesceller]

Wondering if this is necessary to export again the Cargo path on Windows (echo "##vso[task.setvariable variable=PATH;]%PATH%;%USERPROFILE%\.cargo\bin"). Not sure which one Windows is using. Or simply try to display the toolchain before cargo to be sure.

[quentinlesceller]

Found the reason https://github.com/rust-lang/rustup/issues/2203