VM Stack slow

[obscuren]

The current implementation of the vm.Stack is slow because Pop uses the slice delete trick.

• Implement a simple stack using append & a stack ptr
• Remove Popn and swap all x,y := st.Popn() where x = S'2 and y = S'1

Before (GT)

x, y := stack.Popn()

if y.Cmp(x) > 0 { /* ... */ }

After

x, y := stack.pop(), stack.pop()

if x.Cmp(y) > 0 { /* ... */ }