Implement a `trap()` function

The trap() function issues an illegal instruction on supported architectures, falling back to calling abort() elsewhere. It is semantically equivalent to the llvm.trap and __builtin_abort() intrinsics.

On some architectures, issuing an illegal instruction is cheaper than calling an abort function.

The trap function can be used to implement the abort() function by enabling the trap-via-abort feature. This feature does nothing if the architecture is unsupported.

Currently supported architectures for trap:

• x86_64
• i686 (x86)
• aarch64
• arm
• wasm32
• wasm64

On nightly, the trap function wraps the core::intrisnics::abort() function.

Additional fixes:

• Use doc(cfg(...)) to document feature requirements
• Properly configure feature flags for docs.rs
• Drop rustversion dependency in favor of detection in build.rs script.