encode applies to buffers

[swiing]

encode/decode are not features of BitArray. Instead, they are features of ArrayBuffers.

In the current main branch, encode exposes the internals of the BitArray implementation: should we, for whatever reason in the future, want to use buffers of length multiple of 1, or multiple of 8 (or whatever different than today), this would be a breaking change to the encoding, which is undesirable.

The thing is that we are not actually encoding/decoding bit arrays, but rather the buffer bound to the array. So let's be clear about that, and move the encode/decode away from the BitArray.

Note 1: this issue elaborates on #1.
Note 2: in order to implement, it may help to allow instantiating a bit array, given a buffer (back to square 1!).