Add support for EIP-1193 (and drop legacy Web3 support?)

https://github.com/ethers-io/ethers.js/blob/76a8e503dd47b8da1961fb85a6ca28d007f49585/src.ts/providers/web3-provider.ts#L36-L47

From EIP-1193:

ethereum.send(method: String, params?: Array<any>): Promise<any>;

---

The problem here is that ethers is seeing .send on the provider and then treating it as having the following signature:

send?: (request: any, callback: (error: any, response: any) => void) => void

when in reality it has this signature (per EIP-1193):

ethereum.send(method: String, params?: Array<any>): Promise<any>;

This of course leads to failure for two reasons:

1. ethereum.send expects two parameters, method and params, rather than a single JSON-RPC request object.
2. ethereum.send returns the result in a promise, and ethers is waiting for the second parameter (callback) to be called (which will never happen).

---

This is biting me when I try to use my browser-extension provider with a dapp that uses ethers because ethers calls into my window.ethereum.send incorrectly and thus the app fails to work with my extension.

[ricmoo]

Sorry, forgot to tag the commit https://github.com/ethers-io/ethers.js/commit/56af4413b1dd1787db68985e0b612b63d86fdf7c with this issue.

Now that the EIP-1193 specification is backwards compatible, it is much safer to add EIP-1193 support without breaking anyone in the meantime.

The Web3Provider should now work with all versions of Web3 as well as an EIP-1193 compatible provider. It will favour EIP-1193 using Promises, followed by sendAsync and send, using the Web3 callback signature.

[ricmoo]

Closing this now, but if anyone has any issues, please re-open or create a new issue.

Thanks! 😃