README: clarify that format m: command not found is unaffected by numeric precision limits.
Remove some old FIXMEs
Update test/common to handle errors in the test function even when not explicitly testing for errors
Improve the autotester to handle tests where pack(unpack(d)) ~= d by design (e.g. rounding)
Add regression tests for B#21

Three tests are added, checking both the reported bug (certain values to
format p will crash vstruct) and the underlying bug that causes it
(fractional values to formats i and u are not truncated properly).
Fix B#21