Cannot easily round-trip values anymore

[apoelstra]

In #266 (commit 10, 674b272) we changed Value::from_*bits to take a BitIter rather than a generic boolean iter.

However, our Value::iter_* methods return an opaque iterator type. This means that you can't serialize a value into bits and then immediately de-serialize it using this interface. (In fact, there is no good way to do this other than manually constructing a vector from the bits then converting that to a bititer.)

This is a pain point when trying to write a regression fuzz test for Value.