Add `copy-framework`

As mentioned in #188, this command can be used to

1) Copy a framework into an app bundle
2) Strip all unwanted architectures (think `x86_64` slices when building for ARM)
3) Re-sign the framework as needed

This changeset is now functional.

What's left to do:

- [x] Read architectures in the binary from `lipo`
- [x] Actually copy the frameworks that are passed in
- [x] Tests
Sorry if I confused with my comments about `ColdSignal.lazy`, because this _does_ need to be wrapped within it—otherwise, we're executing `binaryURL` eagerly, before the signal has even been started.

But it's enough to wrap _only_ this call, and follow other operators afterward:

```swift
return ColdSignal.lazy {
		return .fromResult(binaryURL(frameworkURL))
	}
	.mergeMap { … }
```