x64

Though we include Boost JSON as a dependency for benchmarking purposes, we do not include it as part of our benchmarks currently.

Essentially introduce const where appropriate for the public types exposed by the api.

This request intentionally only targets some functions since we don't have stable &mut in const contexts along with a bunch of other const stuff that is still nightly.

I don't expect that iced will ever become 100% const, that's unreasonable to assume due to the fact iced needs liballoc around the crate.