hdkey: save seed for use in multiple derivations

[pro-wh]

our wallet wants to derive several accounts at a time. it's more efficient to do the expensive part of stretching the mnemonic into a seed once and allow the library user to get multiple paths' keys from that seed.

oasis-sdk/client-sdk/ts-web/core/src/hdkey.ts

Lines 33 to 35 in 8039a75

	const seed = await mnemonicToSeed(mnemonic, passphrase);
	const key = HDKey.makeHDKey(ED25519_CURVE, seed);
	return key.derivePath(`m/44'/474'/${index}'`).keypair;

Originally posted by @lukaw3d in oasisprotocol/wallet#1454 (comment)

[lukaw3d]

Quick attempt at profiling m/44'/474'/0' ... m/44'/474'/47':

https://github.com/oasisprotocol/oasis-wallet-web/blob/8feb1357d44afc804f1a86b2a88c581152faf1a0/src/app/state/importaccounts/saga.ts#L70-L85

mnemonicToSeed: 841ms
makeHDKey: 183ms
derivePath: 519ms

[pro-wh]

interesting, mnemonicToSeed from bip39 looks like it uses @noble sha512 in pbkdf2 https://github.com/bitcoinjs/bip39/blob/v3.1.0/src/index.js#L41 (c: 2048, whatever that parameter is)
derivePath calls derive, and derive calls makeHDKey, and the stuff outside makeHDKey doesn't look like it ought to take 300+ms
and makeHDKey uses js-sha512, if I were to count it out, three calls per path