Change default optimizations pass to focus on code size

[HCastano]

This PR changes the default optimization option passed to wasm-opt from -O3 to -Os-Oz.
This saves us around 0.2-0.3K according to these rough benchmarks.

The default wasn't set to -Oz as I saw no noticeable size improvements, and it just
takes more time to run. However, you could also argue that since contracts are in general
quite small the extra time for -Oz is negligible so we should enable it anyways.

While I could be convinced of this, I'm not sure what hardware other people build on, and
the negative impact the change to -Oz could have for them.

[athei]

We should invest everything in making contracts as small as possible. Any contract above 50KB wouldn't be viable to run on a parachain anyways. So the optimization time should be negligible. In favor of setting it to -Oz.

[cmichi]

Thank you!

Could you add a ### Changed subsection to the Unreleased section of our CHANGELOG.md: https://github.com/paritytech/cargo-contract/blob/master/CHANGELOG.md ?

[HCastano]

Thank you!

Could you add a ### Changed subsection to the Unreleased section of our CHANGELOG.md: https://github.com/paritytech/cargo-contract/blob/master/CHANGELOG.md ?

I like that a CHANGELOG is kept. However, there's gotta be a way to automate this, no? Maybe @s3krit can share some of his automation secrets 👀