Add ARM build options and instructions

[dantrevino]

Cargo.toml changes:

• made sha2 and sha2-asm "optional" dependencies.
• moved [features] section to the bottom for readability
• sha2-asm is added to the "default" build feature
• sha2 is added to a new "aarch64" build "feature"

Documentation changes:

• cargo build continues to work the same way!
• updated README.md to reflect build instructions and cautions for aarch64 support (cargo build --features "aarch64" --no-default-features).
• added a README-CROSS-COMPILE.md - build instructions for cross-compiling to low memory devices.

fixes #1278

[CLAassistant]

All committers have signed the CLA.

[kantai]

This looks great to me @dantrevino, thanks for putting this together!

For our part, I think we want to make it so that our cargo build is easily amenable to cross-compilation, and your changes to Cargo.toml (and the accompanying instruction in README.md) serve that purpose well. However, we think that cross compilation instructions and support probably shouldn't be in the main repo itself, and would prefer to link out to them as a community supported documentation.

So this is my request: can you drop README-CROSS-COMPILE from this PR, and instead host it somewhere (perhaps your own GitHub repo?) that we can link out to from README.md ?

[kantai]

This looks great to me, thanks for the updates @dantrevino !

Can you merge the latest master into your branch? I want to make sure that the CI will pass on this PR.

[dantrevino]

Doh! Sorry @kantai. Done.

[hstove]

Would love to see this merged if possible. clarity-js-sdk allows you to build from source, but since these changes are in a forked repo, you can't easily build from source using a git branch.

We have a forum user trying to setup clarity-js-sdk on arm64, and I think they'd have to fork clarity-js-sdk to get this temporarily working with Dan's repo.

[jcnelson]

LGTM! Thanks @dantrevino!

[blockstack-devops]

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