Make WasmModule free it's own memory, avoid mixing stack and
heap allocations in tests. This fixes several memory leaks.

Fix several signed compare issues.
Fix several floating point warnings.

Don't setup heap as external, as then the GC can't collect it.

Disable some tests that fail under ASAN.

R=ahaas@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1538543002

Cr-Commit-Position: refs/heads/master@{#32948}