Symbolic memory

[ahuoguo]

After a discussion with @butterunderflow, we decided to first put the concrete/symbolic memory together.

TODO:

• correct behavior for concrete memory
• implement sym-load sym-store
• test offset (we dont need to care abt alignment since it doesn’t have semantic effect)
• memory.size and memory.grow

[butterunderflow]

I'm going to merge this. Current impl has some other issues, but not solely related to symbolic memory.