Support if let guards in pattern matchings

[maximebuyse]

Guards in pattern matchings are supported in Rust but not necessarily in hax backends. We want to support them, and even a generalization with if-let guards (if-lets in guards are a nightly feature in Rust). They should be rewritten by a dedicated phase (see #806).

[W95Psp]

@maximebuyse can you add a bit more info here? e.g. link to Rust manual for if let, the fact it's nightly, also that we want to support "simple" if guards but also if let chains in the future (we should btw open new issues for those)

Oh and one important point is that we do if lets first because it's helpful for #806