Implement loading of prebuilt macros

[mkaput]

Allow macro developers to cross-compile their macros for all relevant platforms and include generated binaries in package tarballs during scarb package. The goal of this task is to teach Scarb to look for such binaries in package sources and prefer it over compiling macros locally.

Platform support

Binaries should be produced for all platforms supported by Scarb, and Scarb should attempt to load them on all of these:

• aarch64-apple-darwin
• aarch64-unknown-linux-gnu
• x86_64-apple-darwin
• x86_64-pc-windows-msvc
• x86_64-unknown-linux-gnu

What determines whether package contains prebuilt binaries?

Any Scarb package that contains a target/scarb/cairo-plugin directory in tarball is considered as one containing prebuilt binaries. This name was chosen because it nicely hides inside Scarb’s/Cargo’s standard build file directory, which is very likely to already be avoided in source repositories and thus shouldn’t conflict with package sources.
Within this directory, binaries for all platforms will follow the following naming pattern: ${PACKAGE_NAME}_v${PACKAGE_VERSION}_${TARGET}.${so|dylib|dll}

Loading

When loading procmacro package that is present in the allowlist (#1769), Scarb will look for the /target/scarb/cairo-plugin directory and will attempt to load a matching binary if present. In any case this loading process fails, Scarb will always fall back to source-code compilation. This will happen silently without any warnings.