Crypto tweaks
Removed test that generates EC key pair using node's crypto API directly.
Due to test implementation it used a code path that attempted to infer the
elliptic curve from the private key size but Node doesn't include significant
bytes that are 0. This would cause key size inference to fail.
We test with a dynamic key using our "createKeyPair" method so above test was
redundant.
Also added code to pad the Node key when we generate our own key internally.
This isn't absolutely necessary because we always provide the public key point
rather than computing from the private key. The public key is encoded
in a format with fixed length coordinates so curve inference works correctly.
But the truncated private key is a bit weird and could lead to future
confusion. We now pad to full key length just to be safe.
Fixes #276
