Env: add `Bitset`

What

This MR does four things:

• changes the encoding of Tezos_base.Bitset.t from Data_encoding.n to Data_encoding.z
• adds an error to Bitset and slightly improves the doc-strings
• puts Tezos_base.Bitset in the environment, thus providing it to the protocol, and remove the now redundant Bitset module from the protocol
• move the tests of the Bitset module from the protocol to the lib_base

Why

There is an unnecessary duplication of the Bitset module.

However, the two modules used different encodings (n in the shell and z in the protocol), so it was needed to unify them. Changing the encoding in the protocol may be problematic, while changing the encoding in the shell should not be. See thread for details.