Better control over error locations for strict contracts

[jneem]

When composing lazy contracts, the reported error position is typically the innermost error: { a = "hi" } | { a | String } will blame just the a and not the whole record. This is usually the correct behavior, I think.

When building up an eager contract by recursively calling std.contract.check on other eager contracts, a contract failure will blame the outermost contract. This is inherent to the signature of std.contract.check, as the inner calls to std.contract.check can only return error messages; the location tracking is only performed at the call to %contract/apply%.

Maybe error case std.contract.check could be modified to return { message, notes, location | optional | Dyn } (i.e. with the new location parameter), where location is an opaque value representing the error location. Then the location could be threaded through recursive calls to std.contract.check, allowing the contract author to decide on the best error location. Maybe location should be locations, an array of locations and error message, to reflect that there can be multiple diagnostic locations tagged with different messages.

Related to #1432, in that they could use the same mechanism for tracking locations.