Consider obscuring seed from Debug implementation of Mnemonic

[TheButlah]

Its a common footgun that secrets get leaked in log messages when types containing private secrets implement Debug, whose implementation actually prints the secret.

bip39 should instead do something like:

impl fmt::Debug for Mnemonic {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
		f.debug_struct("Mnemonic")
			.field("lang", &self.0.language())
			.finish_non_exhaustive()
	}
}