Prevent outside-bounds crash when actually zeroing out heap

[stskeeps]

This one might need a bit closer review

While porting tinygo to TrueBit, I hit that it was unable to zero the heap up to the end of memory.size as reported by the WASM VM, getting a out of bounds error

Memory_size in the following is -memory-size as provided to the interpreter - which I understand as making the task able to access up to 2^(memory_size)

When the TrueBit VM is initialized, a calculation is made to GROW wasm memory with 2^(memory_size - 13) pages (of 65536 byte size each) ---

but SETMEMORY only with 2^(memory_size) is done (actually allocates the memory array)!

And then when memory.size is asked by a program, 2^(memory_size - 13) is returned, which is interpreted as 2^(memory_size - 13) * 65536

Example problem, memory_size = 25

2^25 = 33554432 (32mb)

2^(25-13)* 65536 = 268435456 (256mb)

This patch makes GROW amount of 2^(memory_size) / 65536 pages instead, aligning with what's actually being allocated