Remove direct dependency on elliptic-curve crate

Using the re-export from p256 ensures we always use the correct version.

src/p256.rs

@@ -1,5 +1,5 @@
 use bech32::{ToBase32, Variant};
-use elliptic_curve::sec1::{FromEncodedPoint, ToEncodedPoint};
+use p256::elliptic_curve::sec1::{FromEncodedPoint, ToEncodedPoint};
 use sha2::{Digest, Sha256};
 use std::convert::TryInto;
 use std::fmt;