Support string literals

[Nadrieril]

Thanks to https://github.com/hacspec/hax/pull/649, hax supports string literals as they show up in MIR.

Fixes #72

[Nadrieril]

The problem with this is that aeneas does not have a pretty error path where it uses Literal, so to update aeneas I have to either implement it correctly for all backends or panic.

EDIT: I have been told about OCaml exceptions and aeneas supports those.