Update tiny-bip39 to v0.8 (paritytech#7539)

It would improve secret zeroization due to
maciejhirsz/tiny-bip39#22, and would also remove
one of the points where we depend on `failure` crate, which is
deprecated (see rust-lang-deprecated/failure#347)

Author: Kirill Pimenov

Cargo.lock

@@ -26,7 +26,7 @@ hash256-std-hasher = { version = "0.15.2", default-features = false }
 base58 = { version = "0.1.0", optional = true }
 rand = { version = "0.7.3", optional = true, features = ["small_rng"] }
 substrate-bip39 = { version = "0.4.2", optional = true }
-tiny-bip39 = { version = "0.7", optional = true }
+tiny-bip39 = { version = "0.8", optional = true }
 regex = { version = "1.3.1", optional = true }
 num-traits = { version = "0.2.8", default-features = false }
 zeroize = { version = "1.0.0", default-features = false }