lucet-runtime: heap size limit is hardcoded to 1 MB

[jedisct1]

The max heap size defaults to 1 MB:

    fn default() -> Limits {
        Limits {
            heap_memory_size: 16 * 64 * 1024,
            heap_address_space_size: 0x200000000,
            stack_size: 128 * 1024,
            globals_size: 4096,
        }
    }

We don't seem to ever set it to what is stored in the wasm module, effectively limiting all applications to 1 MB.

[ghost]

Closed by #77