Align keyrings destroy / dispose API

[mikesposito]

Currently, the type we use for Keyrings does not include a "destroy" function. On the other hand, Ledger and Trezor keyring have a function for this and is named dispose for Trezor and destroy for Ledger.

At the moment Ledger and Trezor keyrings are not implementing the Keyring type, but would be beneficial to:

• Add the destroy method on Keyring type
• Rename Trezor's dispose method to destroy
• Trezor keyring should implement Keyring type
• Ledger keyring should implement Keyring type

See also original discussion: MetaMask/KeyringController#233 (comment)