The js backend should better handle OOM

[hhugo]

The reserve function in loader.js doesn't check that there is enough memory before returning.
This can result in the following error when accessing the "reserved" memory.

RangeError: offset is out of bounds

This was spotted using hacl-star with large input.

• consider dynamically growing the memory buffer (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Memory/grow)
• make reserve raise a proper OOM error.