Add support for specifying arch

[stemann]

E.g., for bundling for i686 on x86_64.

Also,

• Added support for being explicit about arch (if arch is not explicitly specified in PackageBundler.toml juliaup.channel field.
• Added platform-dependent IntType to TestPackage, where Serialization fails to deserialize if WORD_SIZE differs from bundling arch to install arch.

[MichaelHatherly]

if arch is not explicitly specified in PackageBundler.toml juliaup.channel field

I'm not too sure I understand this comment. So is this a feature that can be used instead of just specifying the same kind of information in the environment's juliaup.channel field? If so, why can that just be used? I'm guessing I'm missing something here.

[stemann]

if arch is not explicitly specified in PackageBundler.toml juliaup.channel field

I'm not too sure I understand this comment. So is this a feature that can be used instead of just specifying the same kind of information in the environment's juliaup.channel field? If so, why can that just be used? I'm guessing I'm missing something here.

The idea here is to re-use the environment across arch’s - i.e. to just have the environment juliaup.channel specify, e.g., lts, and then call instantiate/bundle with, e.g., arch=:i686 when bundling for i686-w64-mingw32 (and have PackageBundler do the arch naming mangling to make it use the juliaup channel lts~x86 instead of just lts)

[stemann]

Updated wrt. #101 ; the included dev container should also make it possible to test this on x86_64-linux-gnu (bundling for i686-linux-gnu).