optimize + benchmark muloverflow

[holiman]

Adds a benchmarks and simplifies/improves MulOverflow a bit

name                          old time/op    new time/op    delta
MulOverflow/single/uint256-6    25.1ns ± 3%    23.0ns ± 1%  -8.22%  (p=0.016 n=5+4)
MulOverflow/single/big-6        77.0ns ±13%    75.7ns ± 4%    ~     (p=0.937 n=5+5)

name                          old alloc/op   new alloc/op   delta
MulOverflow/single/uint256-6     0.00B          0.00B         ~     (all equal)
MulOverflow/single/big-6         0.00B          0.00B         ~     (all equal)

name                          old allocs/op  new allocs/op  delta
MulOverflow/single/uint256-6      0.00           0.00         ~     (all equal)
MulOverflow/single/big-6          0.00           0.00         ~     (all equal)

[holiman]

Actually, we might need to be careful here, I'm not sure this is correct if the same backing slice is used for input.. is it?

[holiman]

Actually, we might need to be careful here, I'm not sure this is correct if the same backing slice is used for input.. is it?

Nah that's fine, the res from umul is crisp and clean