v2: prohibit string inputs, add explicit method#106

Uint8Array [now has fromHex](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/fromHex). Right now we accept Hex strings or Uint8Array. It is safer to only allow Uint8Array inputs by default. String encoding is useful. We can add the following api: ```js sha256(new Uint8Array(5)); sha256.fromHex('cafe'); sha256.fromUtf8('cafe'); ```