Contract variables can be declared in the `requires` clause and
can be referenced both in `requires` and `ensures`, subject to usual
borrow checking rules.
This allows any setup common to both the `requires` and `ensures`
clauses to only be done once.
This allows the optimiser to properly eliminate contract code
when runtime contract checks are disabled.
It comes at the cost of having to recompile upstream crates
(e.g. std) to enable contracts in them. However, this trade
off is acceptable if it means disabled runtime contract checks
do not affect the runtime performance of the functions they annotate.
With the proper elimination of contract code, which this change
introduces, the runtime performance of annotated functions
should be the same as the original unannotated function.
Refactor contract HIR lowering to ensure no contract code is
executed when contract-checks are disabled.
The call to contract_checks is moved to inside the lowered fn
body, and contract closures are built conditionally, ensuring
no side-effects present in contracts occur when those are disabled.
