Change dense PI construction fn in Composer

There's no other way to enable easy tests for the
users that develop gadgets that is not implement
the Gadget as if it was a Circuit by itself which is
nonsensical.

So this changes the fn visibility to `pub` for
`StandardComposer::construct_dense_pi_vec`.

Resolves: #461

src/constraint_system/composer.rs

@@ -90,7 +90,7 @@ impl StandardComposer {
 
     /// Constructs a dense vector of the Public Inputs from the positions and
     /// the sparse vector that contains the values.
-    pub(crate) fn construct_dense_pi_vec(&self) -> Vec<BlsScalar> {
+    pub fn construct_dense_pi_vec(&self) -> Vec<BlsScalar> {
         let mut pi = vec![BlsScalar::zero(); self.n];
         self.public_inputs_sparse_store
             .iter()