Add multiplexer chip (#23)

It is now possible to perform a mux between two points or between two non-identity points.
`mux(choice, left, right)` will return `left` when `choice=0` and `right` when `choice=1`.
`choice` must be constrained to `{0, 1}` outside the gate.

It is no longer needed to expose `from_coordinates_unchecked`.

halo2_gadgets/src/ecc/chip.rs

@@ -48,7 +48,7 @@ impl EccPoint {
     /// Constructs a point from its coordinates, without checking they are on the curve.
     ///
     /// This is an internal API that we only use where we know we have a valid curve point.
-    pub fn from_coordinates_unchecked(
+    pub(crate) fn from_coordinates_unchecked(
         x: AssignedCell<Assigned<pallas::Base>, pallas::Base>,
         y: AssignedCell<Assigned<pallas::Base>, pallas::Base>,
     ) -> Self {

halo2_gadgets/src/utilities.rs

@@ -12,6 +12,7 @@ use std::ops::Range;
 pub mod cond_swap;
 pub mod decompose_running_sum;
 pub mod lookup_range_check;
+pub mod mux;
 
 /// A type that has a value at either keygen or proving time.
 pub trait FieldValue<F: Field> {