[squashme] replace struct CCainCode with a typedef uint256 ChainCode

libsecp256k1's API changed, so update key.cpp to use it.

Libsecp256k1 now has explicit context objects, which makes it completely thread-safe.
In turn, keep an explicit context object in key.cpp, which is explicitly initialized
destroyed. This is not really pretty now, but it's more efficient than the static
initialized object in key.cpp (which made for example bitcoin-tx slow, as for most of
its calls, libsecp256k1 wasn't actually needed).

This also brings in the new blinding support in libsecp256k1. By passing in a random
seed, temporary variables during the elliptic curve computations are altered, in such
a way that if an attacker does not know the blind, observing the internal operations
leaks less information about the keys used. This was implemented by Greg Maxwell.

Pagelocker is only needed for secure (usually wallet) operations, so don't make
the zero-after-free allocator depend on it.

Instead of manually tweaking the deterministic nonce post-generation,
pass the test case number in as extra entropy to RFC6979.

This was added a while ago for testing purposes, but was never intended to be
used. Remove it until upstream libsecp256k1 decides that verification is
stable/ready.

Remove initialization from vector (as this is only used in the tests).

Also implement SetHex and GetHex in terms of uint256, to avoid
duplicate code as well as avoid endianness issues (as they
work in term of bytes).

- Methods that access the guts of arith_uint256 are removed,
as these are incompatible between endians. Use uint256 instead

- Serialization is no longer needed as arith_uint256's are never
read or written

- GetHash is never used on arith_uint256

We never do 160-bit arithmetic.

If uint256() constructor takes a string, uint256(0) will become
dangerous when uint256 does not take integers anymore (it will go
through std::string(const char*) making a NULL string, and the explicit
keyword is no help).

Introduce new opaque implementation of `uint256`, move old
"arithmetic" implementation to `arith_uint256.

Also add conversion from/to uint256 where needed.

Replace x=0 with .SetNull(),
x==0 with IsNull(), x!=0 with !IsNull().
Replace uses of uint256(0) with uint256().

Also add a stub for arith_uint256 and its conversion functions,
for now completely based on uint256.

Eases step-by-step migration to blob.

Merge branch 'dirty-merge-dash-0.11.0' of http://github.com/infernoman/bitcoin into dirty-merge-dash-0.11.0

icon and splash image changes

Docs Cleanup. Fixes Issue #17