vinyl: rename vy_stmt_env to vy_mem_env

vy_stmt_env doesn't really belong to vy_stmt infrastructure, actually it
isn't used there at all, only created and destroyed. Let's rename it to
vy_mem_env, move it to vy_mem.c, and replace vy_mem->allocator with a
pointer to vy_mem_env. This will allow us to account memory tree blocks
there.

Needed for #934