Upgrade to latest Rust

[zummenix]

Upgrade conrod and all_widgets.rs to latest Rust.

[zummenix]

I'm stuck with this error:

src/envelope_editor.rs:402:32: 402:45 error: type annotations required: cannot resolve
`<<E as envelope_editor::EnvelopePoint>::X as core::ops::Sub>::Output == <E as
envelope_editor::EnvelopePoint>::X` [E0284]

I don't know maybe rust-lang/rust#22036 is relevant.

[zummenix]

Possible solution is to add dummy function:

fn dummy_sub<T: Float>(max: T, min: T) -> T {
    max - min
}

[mitchmindtree]

Thanks for this, I've been running into a few strange errors with assoc types like this, hopefully things will clear up over the next couple weeks :)

I've added your suggestion as a temp fix with a note in src, will PR and merge it in a minute.