Updated to newer version of Rust

[MGlolenstine]

• fixed code and made it compile. (There was a lot of errors because 'usize' moved from u64 to u128)
• cleaned up Markdown documents.
• updated dependencies

[MGlolenstine]

All build checks failed because of the rustup problem. I think that the CI configuration should be updated.

[zicklag]

I'm new to flutter and flutter-rs, but here's my experience testing this PR. 🙂

On Ubuntu 20.04, building an app using the winit backend will flash the window quickly before it immediately crashing afterwards. It's not even getting to the winit event loop processing. This happens for me with the flutter demo app release on GitHub, though so that might not be this PR, but something that was already happening with the winit backend. This does fix the winit backend compile problems, though.

flutter_glfw works for me though, other than #155 which definitely existed before this PR.

[tojocky]

any chance this to be merged?

[MGlolenstine]

@tojocky I think that the CI will have to be fixed first.
Somebody reported that this backend isn't working for them either.
If a library switch is possible, I'd suggest a switch to a more updated RID.

[tojocky]

I'd suggest a switch to a more updated RID

it seems to be based on message store exchange, It is safe but it has a cost. I would prefer to have more direct access.

[MGlolenstine]

You might like the alternative nativeshell, that's been mentioned before over here.

[tojocky]

nativeshell looks interesting. I will stick with direct ffi interface