We would like to move the witness module to `primitives` but there is
a bunch of usage of `VarInt`.
Introduce a module that does the encoding and decoding instead, this
code is internal so put it in `internals`.
Note we add an unused public `MAX_ENCODABLE_SIZE` variable that is
commented with an issue link. Done like this because its quite
important that we see to it and it makes it clear that we are not and
we know about it.
https://github.com/rust-bitcoin/rust-bitcoin/issues/3264
Tobin C. Harding