This implements a macro for implementing the BasepointTable trait, and
uses the macro to create basepoint table types. The default table
still uses radix-16 representation and is ~30KB in size.  The new
table types, and their memory usage and additions required per
basepoint multiplication are:

 * `EdwardsBasepointTableRadix64`: ~120KB, 43 additions
 * `EdwardsBasepointTableRadix128`: ~240KB, 37 additions
 * `EdwardsBasepointTableRadix256`: ~480KB, 32 additions

This is useful for programs/protocol which can do some heuristics or
learning-based approach towards optimising the table size based on the number of
uses of e.g. a public key, the second basepoint in a Pedersen commitment, etc.,
i.e. the first time a public key is used to verify a signature, the usual
variable-time basepoint multiscalar multiplication is used, however after 1000
verifications, the table size is upgraded, and again after 10000 verifications,
etc.

… tables.

Found by the fuzzer in 4f5d2d4.