Pass value_changed to macro_rules! macros

[Aaron1011]

In Rust, variable identifiers in a macro_rules! body are resolved
in the scope of tht body - they cannot see through to the caller's body.
For example, the following code errors:

macro_rules! weird_ident {
    () => { my_ident; }
}

fn main() {
    let my_ident = 1;
    weird_ident!();
}

However, due to a compiler bug (rust-lang/rust#43081),
this kind of code current compiles when a procedural macro is invoked by
a macro_rules! macro. Eventually, this will cause a compilation error.

In the color! and slider! macro, you're currently writing an expression
involving value_changed, and passing it to a procedural macro (html!).
However, value_changed comes from the body of the caller of color!/slider!,
so this code will stop compiling once the compiler bug is fixed.

Fortunately, the identifier of value_changed can be passed into
color! and slider!. This modified code will with the current version of
Rust, as well as future version that causes the old code into an error.

I also ran cargo update to bump the dependencies in your Cargo.lock.
This will allow your crate to continue to compile when the compiler bug is fixed,
as you are depending on some crates (e.g. syn) that have released updates
to address the issue.

Feel free to ask me about any questions you may have. For more details,
see rust-lang/rust#72622