add feature for enabling/disabling inclusion of basepoint tables

[ryankurte]

allows basepoint tables to be excluded to reduce flash usage on embedded devices.

this should have no impact on most users, but will require adding features = [ "basepoint_tables" ] for any folks with default_features=false.

depends on https://github.com/dalek-cryptography/curve25519-dalek/pull/402

cc. @isislovecruft