[PR into tyler/uffd-integration] Fixes uncovered during review of #492

[acfoltzer]

This first commit enables alloc tests for UffdRegion, adds a new test, and fixes some things:

• Instantiates the suite in lucet_runtime_internals::alloc::tests for UffdRegion

• Fixes early return issues in UffdRegion similar to #455

• Adds a test to show that the per-instance heap limit applies to runtime expansions, not just initial instantiation

• Refactors validate_runtime_spec to take the per-instance heap limit as an additional argument. This centralizes the logic for rejecting initially-oversized heap limits, and makes it clearer what's happening in each region's instantiation logic.

• Removes the UffdRegion's assertion that signal stack size is a multiple of page size. Since the user can now control this as a parameter, we reject it gracefully when validating Limits rather than panicking.

[acfoltzer]

This PR now also enables the uffd feature by default, but filters on OS.

Notably, this should get us building and running uffd in Linux CI.

It turns out to be a tremendous pain to enable a feature flag for just one crate within a workspace. The situation is being addressed, but in the meantime I believe the best route forward is to just have uffd on by default for Linux.

[acfoltzer]

Okay, the CI issue is still outstanding, but I'm happy with merging this into your branch, @tyler