update slip39 (#372)

* slip-0039: use AES for obtaining the master seed

* Add rationale behind SLIP-39 design

* slip-0039: add feistel network diagram

* Add more info about choice of finite field and use of pseudorandom permutation.

* Change variables in backward apostrophes to be in italics.

* Add Lagrange interpolation formula

* Add the attack on index encoding to the rationale section.

* Change index and threshold from 6 bits to 5 bits.

* Add alternative passphrase protection methods.

* Replaced Bech32 with Reed-Solomon code over GF(1024). Explained the switch to HMAC-SHA-256. Updated the alternative passphrase protection methods.

* Clean up the different proposals for the master secret derivation function

* Minor fixes and amendments.

* Fix indentation.