Support reusable private key for agreement

[kamyuentse]

Support reusable private key for agreement, introduce a structure Inner, which both used in EphemeralPrivateKey and ReusablePrivateKey.

PS: I close an open this again because I make some mistake in merging and rebasing the commits, apologize for this.

[coveralls]

Coverage remained the same at ?% when pulling e734aa2ebf1c3e798cbfe10cab6e325dc7088682 on kamyuentse:reusable-private-key into d8a13d64e2447e5d0a88394e321a9e862e166267 on briansmith:master.

[davidrusu]

@kamyuentse mind rebasing against master so we can see some green builds (#603)?

[kamyuentse]

@davidrusu rebased!