Feature: add Transaction Replacement support

When using tx.wait() on a transaction made from the sendTransaction() method of a Signer (including non-constant methods on Contract), if the transaction is repriced or cancelled, a TRANSACTION_REPLACED error is thrown along with some extra details of what happened, allowing for appropriate “next steps” to be taken.

Error {
  code: "TRANSACTION_REPLACED",
  // The reason why the transaction was replaced
  // - "repriced" is generally nothing of concern, the 
  //   only difference in the transaction is the gasPrice
  // - "cancelled" means the `to` has been set to the `from`,
  //   the data has been set to `0x` and value set to 0
  // - "replaced" means that the transaction is unrelated to
  //   the original transaction
  reason: "repriced" | "cancelled" | "replaced",
  // This is a short-hand property as the effects of either a
  // "cancelled" or "replaced" tx are effectively cancelled
  cancelled: (reason === "cancelled" || reason === "replaced"),
  // The TransactionResponse which replaced the original
  replacement: [ the replacement transaction response ]
  // The TransactionReceipt of the replacement transaction
  receipt: [ the receipt for the replacement transaction ],
}

This should be supported