Build Regex from static string in a const way

[ValentinColin]

Describe your feature request

Regex are often hardcoded, so it would be nice to be able to build a Regex from a static str in a const context to avoid handling error that will never happens. The build step of the regex we be done a compile time and not at runtime.

Example with the crate http: http::header::HeaderValue::from_static(&'static str).

[BurntSushi]

• #607
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