[API change] add struct minmea_float w/ int_least32_t

This is an API breaker that incorporates the following big changes:

1. The (value, scale) float, as used in minmea, is a compound type. It
should be declared and used as such.

2. Some platforms, notably avr-gcc, have 16-bit ints. Make life easier
for them by using int_least32_t for floating point values.

3. As a side effect, the following functions were renamed:

* minmea_coord -> minmea_tocoord
* minmea_float -> minmea_tofloat

Migration guide for user code (easy unless you do fancy stuff):

1. Replace calls to minmea_{coord,float}.

2. Replace int32_t with int_least32_t in user code if your platform
requires this.
example.c: update for the new API
example.c: fix printed frame names
README.md: reflect the API changes