Consider ARMv7 support

[jkeiser]

Currently we only support ARMv8. @easyaspi314 submitted a PR for ARMv7, but we need to figure out how to test it in CI and check its performance.

Alternately, we could include it as an unsupported implementation (off by default). The user could turn it on with -DSIMDJSON_IMPLEMENTATION_ARM64=1. That might be easier than working this stuff out, and keep the code from completely rotting.