Make FlexZeroVec's ULE type be FlexULE

[sffc]

Currently it is &[u8] which makes it hard to work with in APIs that give back ULE types. It should be &FlexULE so that we can add a function to convert it into a usize.

Also use this opportunity to clean up call sites around zerovec to use the new helper function.

[sffc]

CC @Manishearth OK?

[Manishearth]

+1