feat: refactor inner-product prover (#57)

The inner-product prover is a bit clunky, as it requires the range
prover to set up a struct, perform iterative rounds, and then parse out
the proof elements it needs. This also requires consistency checks that
it would be nicer to avoid.

This PR moves the inner-product prover into the range prover, which
simplifies things at the cost of a longer prover function.

It also fixes some [vector
allocations](https://github.com/tari-project/bulletproofs-plus/blob/d9d0cc9063f85684179908569227dda251981751/src/inner_product_round.rs#L111-L112)
that were much bigger than necessary.

Closes #61. Closes #55. Supersedes #56.

src/errors.rs

@@ -20,4 +20,7 @@ pub enum ProofError {
     /// Invalid `Blake2b` hash operation
     #[error("Invalid Blake2b")]
     InvalidBlake2b,
+    /// Internal size overflow
+    #[error("Internal size overflow")]
+    SizeOverflow,
 }