namespace barretenberg

[codygunton]

Right now I think there is no real utility in having this namespace, and it should be split up, for instance, into ecc, polynomial, srs, bn254, grumpkin, secp256k1 and secp256r1. This is not urgent, but it has come up recently because of the asymmetry between barretenberg::fr/fq and grumpkin::fr/fq. Functions used for bn254 and grumpkin are being put in the barretenberg namespace, which is sort of awkward. Also, it's not like "barretenberg" is convenient to type, and it would surely be clearer people newly looking at the code to see bn254::fr in various places rather than barretenberg::fr.

What we should do is put everything in /src/barretenberg into the namespace barretenberg, and then we should move some things such as barretenberg::fr farther up the namespace chain.

[codygunton]

Follow-up: Charlie raised this on a call with me last week. He points out that everything in src/barretenberg should be in barretenberg namespace for when external consumers use Bb as a C++ library.

[codygunton]

Transferred to #794