Use switch instead of if when evaluating curves

Use switch instead of if when evaluating curves

Previously, the code would do:

    REQUIRE(alg == CURVE1 || alg == CURVE2);

    [...]

    if (alg == CURVE1) { /* code for CURVE1 */ }
    else { /* code for CURVE2 */ }

This approach is less extensible and also more prone to errors in case
the initial REQUIRE() is forgotten.  The code has been refactored to
use:

    REQUIRE(alg == CURVE1 || alg == CURVE2);

    [...]

    switch (alg) {
    case CURVE1: /* code for CURVE1 */; break;
    case CURVE2: /* code for CURVE2 */; break;
    default: INSIST(0);
    }