Zeroize keys on drop (#156)

(cherry picked from commit 468e0690d7)

Cargo.lock

@@ -1017,6 +1017,18 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "synstructure"
+version = "0.12.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b834f2d66f734cb897113e34aaff2f1ab4719ca946f9a7358dba8f8064148701"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "unicode-xid",
+]
+
 [[package]]
 name = "tempfile"
 version = "3.2.0"
@@ -1248,3 +1260,18 @@ name = "zeroize"
 version = "1.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "81a974bcdd357f0dca4d41677db03436324d45a4c9ed2d0b873a5a360ce41c36"
+dependencies = [
+ "zeroize_derive",
+]
+
+[[package]]
+name = "zeroize_derive"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a2c1e130bebaeab2f23886bf9acbaca14b092408c452543c857f66399cd6dab1"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "synstructure",
+]