Cryptographic Secret type (#615)

* Adds a type for cryptographic secrets

* default implementations and zeroize documentation

* removes whitespace

Cargo.lock

@@ -67,6 +67,7 @@ dependencies = [
  "serde_json",
  "subtle",
  "untrusted",
+ "zeroize",
 ]
 
 [[package]]
@@ -242,6 +243,7 @@ dependencies = [
  "sk-cbor",
  "subtle",
  "uuid",
+ "zeroize",
 ]
 
 [[package]]
@@ -443,12 +445,30 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "synstructure"
+version = "0.12.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f36bdaa60a83aca3921b5259d5400cbf5e90fc51931376a9bd4a0eb79aa7210f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "unicode-xid",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dcc811dc4066ac62f84f11307873c4850cb653bfa9b1719cee2bd2204a4bc5dd"
 
+[[package]]
+name = "unicode-xid"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f962df74c8c05a667b5ee8bcf162993134c104e96440b663c8daa176dc772d8c"
+
 [[package]]
 name = "untrusted"
 version = "0.7.1"
@@ -561,3 +581,24 @@ name = "winapi-x86_64-pc-windows-gnu"
 version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
+
+[[package]]
+name = "zeroize"
+version = "1.5.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c394b5bd0c6f669e7275d9c20aa90ae064cb22e75a1cad54e1b34088034b149f"
+dependencies = [
+ "zeroize_derive",
+]
+
+[[package]]
+name = "zeroize_derive"
+version = "1.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "44bf07cb3e50ea2003396695d58bf46bc9887a1f362260446fad6bc4e79bd36c"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "synstructure",
+]