Add a hash-to-curve test template

[mmagician]

Then we can remove the following test modules entirely:

• algebra/ec/src/hashing/curve_maps/swu.rs

  Line 144 in 30b02b4

  mod test {

• algebra/ec/src/hashing/curve_maps/elligator2.rs

  Line 151 in 0eeb7cb

  mod test {

and instead add a macro to run randomized tests.

Note: we currently have a macro test_h2c, which reads test vectors (together with expected outputs). This in general won't be available for most curves, maybe we can rename it to test_h2c_suite, and the new one that runs randomized tests be called test_h2c.

@drskalman would you be interested in helping out with this?