Cache HMAC object and fix memory leak

[qmuntal]

This PR improves NewHMAC so it reuses the same EVP_MAC object instead of fetching it every time.

Doing so also fixed a memory leak: the object returned by EVP_MAC_fetch should have been freed using EVP_MAC_free. The new approach doesn't require freeing that object, as it is kept as a global variable to be reused later on.

Thanks to caching, NewHMAC is now ~9% faster:

goos: linux
goarch: amd64
pkg: github.com/golang-fips/openssl
cpu: AMD EPYC 7763 64-Core Processor                
           │   old.txt   │              new.txt               │
           │   sec/op    │   sec/op     vs base               │
NewHMAC-16   4.616µ ± 1%   4.199µ ± 1%  -9.04% (p=0.000 n=20)

[simo5]

LGTM