add script to compute precomputed curves, elliptic.usePrecomputed, tests

[mvayngrib]

the goal is to be able to sacrifice bundle size for performance by choosing to use a precomputed curve at runtime:

const elliptic = require('elliptic');
elliptic.usePrecomputed({
  // require only the ones you need
  p256: require('elliptic/lib/elliptic/precomputed/p256'),
});

also, the validation in the PresetCurve is expensive and unnecessary to do at runtime for pre-defined curves (i think but correct me if i'm wrong!)

this PR incorporates #188 , but i can adjust and rebase if that's merged first

this is kind of a big PR, but we noticed (at Exodus) that elliptic.ec('p256') takes ~1s in React Native on an iPhone Xs, and in that environment the tradeoff of a slightly bigger bundle as a result of bundling a precomputed curve is more than offset by the savings in computation time (~700ms savings)

relevant PR in Exodus: ExodusForks#1

thanks @fanatid for help in figuring out how to do this

[coveralls]

Coverage increased (+0.2%) to 89.282% when pulling dae93e6 on ExodusMovement:mv/precomputed_1 into 71e4e8e on indutny:master.