ensure BitVec deser has correct len/cap

[cpubot]

Problem

It's possible to get BitVec into an invalid state when deserializing under the following conditions:

• Encoded sequence has length zero (for any type T), because all sequence lengths are encoded as u64
• Encoded sequence of bytes where length is < NUM_WORDS

This can trigger a panic, because boundary checks use constants like NUM_WORDS and NUM_BITS, which are meaningless if the inner vec's state is out of sync with them

Summary of Changes

Deserialize impl of BitVec now ensures it has the expected number of words