28/29 word "Sia" recovery phrase support

[ChrisSchinnerl]

We should be able to create a WASM function for this pretty easily.
Every wallet since walrus has been backwards-compatible with siad seeds. siad seeds are 32 bytes of entropy, which are hashed with an integer to derive sequential keys; walrus seeds are 16 bytes of entropy, which are hashed to produce 32 bytes, which are then hashed with an integer. So the only difference is that walrus seeds get hashed an extra time before you use them.

[n8maninger]

We will not support Sia 28/29 seeds in the walletd UI. Instead, we will follow hostd and create a utility to sweep the funds of legacy wallet formats, both Sia seed and v0.3.x siag key files, to a new address.