opaque.go | 3 3 + 0 - 0 !
opaque_test.go | 55 0 + 55 - 0 !
2 files changed, 3 insertions(+), 55 deletions(-)

 [patch] doc: note opaquekeyencrypter can't be implemented (#129)

Also, remove TestOpaqueKeyRoundtripJWE test case, which is expensive
(because it incidentally uses the large default PBES2Count) and isn't
testing reachable code. It can be restored if/when we fix the API for
OpaqueKeyEncrypter so it can be implemented (and possibly pass along
PBES2 parameters).

Fixes #112, updates #114

Note: I tried simply exporting `encryptKey` as `EncryptKey` and adding
parameters, but it wound up getting more involved because we would also
have to export the return type `recipientInfo`, which in turn contains
the unexported `*rawHeader`. For now this documents the status quo
(OpaqueKeyEncrypter doesn't work) and speeds up our tests dramatically
(11s on my machine).