Hello,
It looks we have some duplication between btc_to_satoshi/parse_amount
and satoshi_to_btc/format_amount. These routines both convert between
strings and numbers for Bitcoin amounts, so we should just pick one or
the other and remove duplication.

My vote is in favor of the parse_amount/format_amount pair. The
parse_amount routine, besides having full checks for overflow, invalid
characters, and so forth, have two features that makes it a bit better
than btc_to_satoshi.

The first is an adjustable precision parameter, so the routine can
handle millibits, microbits, and possibly weird altcoins with
different precisions. We use this in the AirBitz wallet. If we wanted
to go with btc_to_satoshi, we would have to add this adjustment.

The other feature is rounding. Not everyone is perfect, and our wallet
occasionally sees URI's with 9 or more digits after the decimal point.
Failing to parse these URI's would be a horrible user experience, so
we need to do something reasonable like rounding. Also, the BIP 0021
standard doesn't say anything about the number of acceptable digits
after the decimal place, so failing to parse these amounts would
arguably be non-compliant. So, parse_amount is the better choice here.

The btc_to_satoshi routine does return a bool, which may or may not be
nicer cosmetic than returning invalid_amount. We can change the
signature of parse_amount if this is a big deal.

-William