abi: dynamically load the dylib at runtime

[miraclx]

Based off #23

Instead of building an intermediate project that loads the dylib to extract data, we can dynamically load the dylib at runtime from inside cargo-near itself.

#31 (comment)

[itegulov]

Awesome, I was unaware that it was this easy to load and call a dylib in runtime!

This change has one unintended side-effect though: we no longer create target/near directory automatically, so writing ABI to target/near/abi.json can fail if the directory is missing. Try running cargo clean && cargo near build --out-dir res --release --embed-abi on the abi example from SDK repo to see what I mean.

[miraclx]

That is correct, nice catch!