stupidgcm: cache C.EVP_chacha20_poly1305()
stupidgcm: cache C.EVP_chacha20_poly1305()

2% performance improvement, almost for free.

gocryptfs/internal/speed$ benchstat old new
name             old time/op   new time/op   delta
StupidXchacha-4   5.82µs ± 0%   5.68µs ± 0%  -2.37%  (p=0.008 n=5+5)

name             old speed     new speed     delta
StupidXchacha-4  704MB/s ± 0%  721MB/s ± 0%  +2.43%  (p=0.008 n=5+5)

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs