Add support for RC4

[qmuntal]

This PR implements the necessary functions to support crypto/rc4.

New benchmark results:

goos: windows
goarch: amd64
pkg: github.com/golang-fips/openssl/v2
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
BenchmarkRC4_128-12      5396026               220.1 ns/op       581.51 MB/s           0 B/op          0 allocs/op
BenchmarkRC4_1K-12        704779              1532 ns/op         668.28 MB/s           0 B/op          0 allocs/op
BenchmarkRC4_8K-12         96313             12660 ns/op         639.48 MB/s           0 B/op          0 allocs/op

[qmuntal]

@derekparker @ueno I'll be taking a long leave soon and I'll like to have this PR as ready as possible to be merged. Could you take a look? Thanks!

[ueno]

LGTM!