high-level API: simplify encoding a whole number of bytes

[dkg]

The high-level encode function is still slightly unergonomic for the common use case of encoding a whole number of bytes.

As far as i can tell, the only errors possible for encode are:

• bits doesn't match length of input
• total encoded value will exceed the ability to allocate RAM

For the common use case it would be nice to have something like:

pub fn encode_full_bytes(input: &[u8]) -> Result(String, ZBase32Error)

or even (treating memory allocation failures as unrecoverable errors):

pub fn encode_full_bytes(input: &[u8]) -> String

(this latter interface is offered by the popular zbase32 crate, so implementing it would make it easier for people to move from zbase32 to this implementation)