MIR: Use Micheline in parser and typechecker

Context

Rewire parser and typechecker to work with Micheline-like structure.

Using an owning Micheline-like construct a-la enum Micheline { ..., App(Prim, Vec<Micheline>, ...), ... } in Rust is rather inconvenient:

1. Allocating many small (<8 elements) vectors is expensive both time- and space-wise, and Micheline basically consists of small vectors.
2. Pattern-matching on Vec is rather cumbersome (deref patterns aren't here yet), and we need to do a lot of pattern-matching in the typechecker.

Thus, instead, a non-owning Micheline struct is used. Allocations are done via the typed-arena crate. Using an arena slightly complicates things (something has to own the arena, and it needs to be passed around), but it's not that cumbersome, partially addresses (1) and completely resolves (2), so it's worth it.

One hiccup is that the parser still allocates small vectors. This can be alleviated to a large extent by handling common cases of 0-, 1- and 2-argument applications in the grammar explicitly, but that will complicate the grammar, and benchmarking is required to gauge the impact, so this is left for future work.

Based on: !10783 (merged)

Part of: #6539 (closed)

Resolves #6360 (closed)

Manually testing the MR

cargo test; this is mostly a refactoring MR; some tests had to be moved around, though, as syntax errors handled previously by the parser now have to be handled in the typechecker.

Checklist

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• Provide automatic testing (see the testing guide).
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