math/big: more tests, documentation for Flot gob marshalling
Follow-up to https://golang.org/cl/21755.
This turned out to be a bit more than just a few nits
as originally expected in that CL.
1) The actual mantissa may be shorter than required for the
given precision (because of trailing 0's): no need to
allocate space for it (and transmit 0's). This can save
a lot of space when the precision is high: E.g., for
prec == 1000, 16 words or 128 bytes are required at the
most, but if the actual number is short, it may be much
less (for the test cases present, it's significantly less).
2) The actual mantissa may be longer than the number of
words required for the given precision: make sure to
not overflow when encoding in bytes.
3) Add more documentation.
4) Add more tests.
Change-Id: I9f40c408cfdd9183a8e81076d2f7d6c75e7a00e9
Reviewed-on: https://go-review.googlesource.com/22324 Reviewed-by: Alan Donovan <adonovan@google.com>