Add benchmarks for generation and fingerprinting

Results with ` i7-3687U CPU @ 2.10GHz × 4`
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BenchmarkECP256Generate	    5000	    765631 ns/op
BenchmarkECP384Generate	     200	  10228670 ns/op
BenchmarkECP521Generate	     100	  17862164 ns/op
BenchmarkRSA2048Generate	       2	 911764954 ns/op
BenchmarkRSA3072Generate	       1	1426658951 ns/op
BenchmarkRSA4096Generate	       1	19010220221 ns/op
BenchmarkECP256Fingerprint	  500000	      3202 ns/op
BenchmarkECP384Fingerprint	  500000	      3304 ns/op
BenchmarkECP521Fingerprint	  500000	      4004 ns/op
BenchmarkRSA2048Fingerprint	      10	 174062887 ns/op
BenchmarkRSA3072Fingerprint	       1	6930198558 ns/op
BenchmarkRSA4096Fingerprint	       1	3772528822 ns/op
~~~~