[RFC007] Extract runtime contract equality out of typecheck (#2130)

* Extract runtime contract equality out of typecheck

This continues the preparative work to migrate the typechecker to the
new AST. One current blocker is that the `typecheck::eq` is used both by
the typechecker and at runtime for contract deduplication; however we
will get rid of it eventually, while still needing to perform contract
deduplication (until the landing of the bytecode virtual machine).

This commit extract the code from `typecheck::eq` for contract
deduplication in a new module, so that the runtime isn't dependent on
`typecheck` anymore. The code is specialized (as it doesn't need to
handle static contract equality), and in order to eliminate the
dependence on `UnifType`, we drop the ability to compare polymorphic
type. The latter is the only reason `typecheck::eq` operated on
`UnifType` instead of `Type`, but the motivation is not very convincing:
how many times do we encounter a type annotation, itself inside a
contract annotation, such that annotation is a polymorphic type AND it
happens to be a candidate for deduplication? Our intuition would be: zero.

This hypothesis will be validated on our working benchmarks
(instrumentation counts each contract deduplication)

* Update core/src/eval/contract_eq.rs

Co-authored-by: jneem <[email protected]>

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Co-authored-by: jneem <[email protected]>