Use consistent byte/char offsets when parsing invoice HRPs

Use consistent byte/char offsets when parsing invoice HRPs

When parsing lightning-invoice HRPs we want to read them
char-by-char, tracking at which offset different fields were. Prior
to this commit this was done first by reading char-by-char and then
by indexing using the byte offset which works for ASCII strings but
fails on multi-byte characters.

This commit fixes this issue by simply always walking byte-by-byte
and rejecting multi-byte characters which don't belong in HRPs.