Benchmark: Costlang.Fold_constants: reduces let x = c in e when c is a constant

This introduces let x = const in e => e[const/x] reduction to Costlang's constant folding transformer Fold_constants.

This optimization is required for the correct benchmark dependency analysis by eliminating some coefficient variables multiplied by 0 of the intercept cases such as addlogadd. It is like

fun size1 -> fun size2 ->
  let a = size1 + size2 in
  coeff * (a * log2 (1 + a)) + const

For the intercept case, size1 and size2 are fixed to 0. However, currently they do not optimize coeff out currently. Instead we have:

  let a = 0 in
  coeff * (a * log2 (1 + a)) + const

This makes the benchmarks of the non-intercept case and intercept case of N_IMul_int and N_IMul_nat ambiguous.

This MR expands let a = 0 and simplifies the above formula to a simple

  const

and resolves the ambiguity of N_IMul_*.