use core::intrinsics::abort() in panic handler and ditch build-std option (compiles std, alloc, core)

[greenhat]

See the Rust -> MASM compilation example at #91 for panic handler.

Remove build-std option in CompilerTest.

[bitwalker]

I discovered that we do not need to use core::intrinsics::abort, instead we can use core::arch::wasm32::unreachable, which has the same effect, but is stable, as opposed to core::intrinsics::abort which is perma-unstable. I've updated the guides in our documentation, but we should probably update everything else that uses unstable intrinsic (or that we held off on updating due to the instability).

For crates which need to compile for both Wasm and non-Wasm targets, we can use conditional compilation to use the wasm32 intrinsic when appropriate, but I think most everything we have that needs it, is only targeting Wasm anyway.

[bitwalker]

@greenhat Do you recall why we haven't made this change yet? I can't remember if we ran into a specific issue, or just didn't have the time for it.

[greenhat]

I tried and ran into - #135 (comment)