Expose memory.grow, memory.size, memory.dataLength

Goal

Unblock work for a full-blown allocator.

Overview

memory_grow are not available through walt, but can be implemented as members of the Memory and Table types.

Acceptance Criteria

This should work

const memory : Memory = { initial: 1 };

function test() {
  memory.size();
  memory.grow();
  memory.dataSize();
}

• Memory.grow()
• Memory.size() current size of memory
• Memory.dataSize length of the data section, always zero until data senction encoding is implemented.

[ballercat]

Implemented by #148

[poeticAndroid]

this would be nice 😃