Provide some wrapper functions to hide all the intricacies of dealing
with Emscrpten heap. We should use these functions instead of calling
C directly.

Note that they ensure that allocated memory is zeroed out before use
as well as after use. This makes it easier to debug initialization
issues, and reduces the likelyhood of sensitive data leakage through
Emscripten heap.

Instead of calling malloc() and free() directly, use our new helpers
to deal with passing byte arrays between JavaScript and Emscripten.

Instead of calling malloc() and free() directly, use our new helpers
to deal with passing byte arrays between JavaScript and Emscripten.

Instead of calling malloc() and free() directly, use our new helpers
to deal with passing byte arrays between JavaScript and Emscripten.

Instead of calling malloc() and free() directly, use our new helpers
to deal with passing byte arrays between JavaScript and Emscripten.

Instead of calling malloc() and free() directly, use our new helpers
to deal with passing byte arrays between JavaScript and Emscripten.

heapAlloc: if malloc() fails then do not zero out the result

heapFree: make this function a no-op to allow its usage in failure
paths where the pointer is null (e.g., returned by failed heapAlloc)

We use a weird check "!!buffer" because JavaScript has weird ideas
about what is null and what is zero. malloc() results are generally
checked with "!ptr" so we invert this check.