Detect bech32 typo position(s)

The bech32 address format has pretty neat error detection functions that could really improve a wallet user experience. See it in action [here](http://bitcoin.sipa.be/bech32/demo/demo.html).

We could add something like:

```c
`wally_validate_bech32(const char *addr, const char *addr_family, uint32_t flags, unsigned char *bytes_out, size_t len, size_t *written
```

where `bytes_out` is be an array with the position(s) of invalid characters.

I'm not sure how this would be implemented.