Ensure that witnet-toolkit version always matches witnet version

[tmpolaczyk]

In the assets of the 1.5.1 release: https://github.com/witnet/witnet-rust/releases/tag/1.5.1 there is a witnet_toolkit binary that says its version is 1.4.3, but that's not true because the toolkit includes the latest changes from 1.5.1.

The issue is that the version was simply not updated in Cargo.toml:

https://github.com/witnet/witnet-rust/blob/253d5b1d0579f8d6e18a80f33059b833b34b3938/toolkit/Cargo.toml#L3

So we should implement some mechanism to prevent this kind of issues in the future. Maybe a command to automatically bump the versions of all the crates, or simply a test that fails if there is a version mismatch between some of the binaries.

I believe this only affects the binaries that are included in the release assets, so witnet and witnet_toolkit. There are many crates in this repository that have a different version number, but that's not a problem because they are not published anywhere, and the only way to add them as a dependency is using the github URL, so the commit hash is the ultimate version number in that case.