Use Buffer.from for NodeJS fromHexString

[dapplion]

@nflaig noted that SSZ uses a browser compatible but slow implementation for fromHexString

ssz/packages/ssz/src/util/byteArray.ts

Lines 32 to 35 in d030130

	for (let i = 0; i < byteLen; i++) {
	const byte = parseInt(hex.slice(i * 2, (i + 1) * 2), 16);
	bytes[i] = byte;
	}

We may use platform specific exports to get around it or have some logic to pick between implementations depending on platform.

Important to note that many consumers of SSZ assume everything is a Uint8Array. Buffer.slice() has different behaviour than Uint8Array.slice, so we must audit all downstream code to prevent surprises.

[nflaig]

We may use platform specific exports to get around it or have some logic to pick between implementations depending on platform.

Another and much simpler solution could be to just check if Buffer.from exists as we do for base64 utils in Lodestar

packages/utils/src/base64.ts#L1

const hasBufferFrom = typeof Buffer !== "undefined" && typeof Buffer.from === "function";

However, if there are more functions that are affected, exports might be a more scalable / robust solution