In this commit, we fix a logic error in our routine for converting a
uint64 to/from base32. Before this commit, we assumed that the max
number of groups was 12. However, the math.MaxUint64 (1<<64 - 1) can
actually consume more than 12 groups with an extra set of bits. Before
this commit, we would panic when attempting to parse an invoice
generated like so:
* addinvoice --amt 1337000 --expiry 99999999999999999
To fix this issue, we modify our logic to expect at most 13 groups.
Additionally, we've added a new test that would panic before applying
this commit.
Fixes#972.
