Try building standard library from source for release builds

[xfbs]

The Leptos book recommends some strategies for keeping WASM sizes small specifically:

• Using opt-level = "z"
• Building standard library from scratch

I think we can (should) try both of these. For the opt-level, it results in a slow experience, which can be mitigated by changing the opt-levels of certain dependencies (specifically fancy-regex) to be 3.

I'm curious as to how much of a size difference this would make (and what the impact would be on the performance).