refactor(shortint): rename generate_accumulator into generate_lookup_table

This renames the generate_accumulator* family of
functions into `generate_lookup_table`.

The reasoning is that we have `generate_accumulator`
which returns a `LookupTable` type, and you use
that with the `apply_lookup_table` function which is not
coherent.

Accumulator was the name we had originaly and consistently
for these, however lookup_table is probably easier to
understand / guess what it is about.
Also, it is the term used in concrete, the other
Zama product.

README.md

@@ -111,8 +111,8 @@ fn main() {
     // f: x -> sum of the bits of x
     let f = |x:u64| x.count_ones() as u64;
 
-    // Generate the accumulator for the function
-    let acc = server_key.generate_accumulator(f);
+    // Generate the lookup table for the function
+    let acc = server_key.generate_lookup_table(f);
 
     // Compute the function over the ciphertext using the PBS
     let ct_res = server_key.apply_lookup_table(&ct_add, &acc);