Make checksums optional

[alexkirsz]

We run the uniffi bindgen with Bazel, and our rules ensure there can be no version mismatch between the Rust and foreign language bindings. This makes checksums purely redundant.

We would love to have the option to disable checksums entirely.

[mhammond]

The checksums are to try and ensure the correct library is loaded at runtime, which the build process can't really ensure. #1789 also notes that the checksums aren't particularly adequate for proc-macros anyway, and I don't see an easy fix for that, so this is a reasonable discussion to have.

[bendk]

This seems reasonable to me. I think it could be a top-level field in uniffi.toml.