Investigate supporting Tahoe-LAFS mutable capabilities

[crwood]

The keys which constitute Tahoe-LAFS mutable capabilities are derived from RSA keys (passed through a custom "tagged" sha256d hash function as per hashutil.py). Investigate how/whether this library could or should support generating equivalent Tahoe-LAFS capabilities without the need to depend on Tahoe-LAFS itself.

[crwood]

Some other reasons to do this:

• The allmydata package isn't really designed or intended to be used as a python library/API.
• Deriving a Tahoe-LAFS capability string currently requires making an mkdir call to the Tahoe-LAFS web API -- which, in turn, requires being connected to a live grid; it would be useful to be able to reconstruct caps "offline" as a pure function, etc.
• Attempting to re-create a Tahoe-LAFS directory that already exists (i.e., by supplying the same RSA key to the mkdir call) throws an UncoordinatedWriteError and may lead to data-loss.
• Another implementation (and common test vectors?) helps verify existing functionality.