Improve and add new --optimize flags for the AssemblyScript Compiler

[torch2424]

Hello! cc @dcodeIO

As we discussed in a few other threads, we discussed the changes in the weekly meeting: https://github.com/AssemblyScript/working-group/issues/58#issuecomment-806073112

I think, as we also agreed, that for a better developer experience for those first learning AssemblyScript, we should: "make --optimize alias to --optimize-performance (-O3) and then make a new flag --optimize-size (-Oz)"

Let me know what y'all think! 😄 🎉