base32ct: add const encoded_len fn

[dodomorandi]

This can be handy to avoid an heap allocation, because it is possible to use the result of the const fn to allocate an array of the right size.

EDIT: I changed a bit the initial proposal in order to make it actually easier to use in const context. Previously encoded_len took a slice (like in the trait), now it takes an usize representing the number of bytes to encode.