Potential memory leak when running as WASM

[aweiker]

In tracking down a memory leak in our code we were able to isolate it down to a few different possibilities (including the upgrade from 1.0.40 to 1.0.41) and it caught my attention that PR #1273 introduced the use of core::hint::unreachable_unchecked() which the documentation states that the compiler may have generated nonsensical machine instructions for this situation, including in seemingly unrelated code, causing difficult-to-debug problems source.

While I haven't had a chance to isolate and provide an easy to reproduce scenario; I thought it worthwhile to ask why this was used instead of the unreachable! macro as the documentation suggests, or worse case just use panic!.

[Manishearth]

#1276 should fix the unreachable_unchecked