add Sign1Message benchmarks

[qmuntal]

This PR adds benchmarks for Sign1Message methods. These benchmarks will be useful later one to do memory and cpu profiling.

These are the results on my Windows laptop (use only for reference):

goos: windows
goarch: amd64
pkg: github.com/veraison/go-cose
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
BenchmarkSign1Message_MarshalCBOR-12              666874              1791 ns/op             692 B/op         17 allocs/op
BenchmarkSign1Message_UnmarshalCBOR-12            368671              3303 ns/op            1592 B/op         28 allocs/op
BenchmarkSign1Message_Sign-12                     631834              1739 ns/op             668 B/op         16 allocs/op
BenchmarkSign1Message_Verify-12                   585108              1910 ns/op             668 B/op         16 allocs/op

For #52

[thomas-fossati]

Looks good (and useful) to me, thanks!

I have inlined a small editorial comment.

[thomas-fossati]

isn't this a dup of zeroSource in conformance_test.go?

[qmuntal]

Good catch!

[yogeshbdeshpande]

Few General comments:
Are you planning to have bench test for sign in other file, then please rename this file to bench_test_sign1.go
Perhaps for future, makes more sense to move the bench test files to a new folder as benchmark and add all the tests there?

[qmuntal]

Few General comments: Are you planning to have bench test for sign in other file, then please rename this file to bench_test_sign1.go Perhaps for future, makes more sense to move the bench test files to a new folder as benchmark and add all the tests there?

I plan to stick with bench_test.go for new benchmarks, as I don't expect that those grow too much.
Also, it is common and idiomatic to keep benchmark tests in the same directory as the code being benchmarked.

[shizhMSFT]

LGTM

[shizhMSFT]

LGTM