Replace uint256/uint160 by opaque blobs where possible

This pull replaces almost all uses of `uint256` and all uses of `uint160` to use opaque byte blobs `blob256` and `blob160` with **only** the following operations: 

- Default initialization to 0, or from a vector of bytes
- Assignment from other blobXXXs
- `IsNull()` compare to special all-zeros value
- `SetNull()` clear to special all-zeros value: Bitcoin needs `IsNull()` / `SetNull()` because we often use the all-zeroes value as a marker for errors and empty values.
- `<` for sorting in maps.
- `!=` `==` test for (in)equality
- `GetHex`/`SetHex`/`ToString` because they're just so useful
- `begin()`/`end()` raw access to the data
- `size()` returns a fixed size
- `GetSerializeSize()` `Serialize` `Unserialize` serialization just reads and writes the bytes
- `GetCheapHash()` this is similar to `GetLow64()` and returns part of the value as uint64_t, for cheap hashing when the contents are assumed distributed uniformy random.

`uint256` (used for proof-of-work mainly), on the other hand, loses all functionality like raw bytes access and serialization. Its memory should not be accessed directly. This is necessary for #888 (big endian support).
`uint160` is completely removed as Bitcoin Core does no 160-bit integer arithmetic.

### Overall steps (see commits)

Even though the diff is huge, I've tried to follow a logical and easy to review process:

0. Introduce `base_uint::SetNull` and `base_uint::IsNull()` as well as other methods that will later be on `base_blob`, to prepare for migration
1. Replace x=0 with .SetNull(), x==0 with IsNull(), x!=0 with !IsNull().  Replace uses of uint256(0) with uint256().
2. Introduce blob256 and blob160 as well as conversion functions   (only needed for blob256, we don't ever compute with uint160).
3. Replace `GetLow64()` with `GetCheapHash()`
4. Rename uint256 and uint160 to blob256 and blob160 except where big integers are really necessary.   For reviewing convenience I separated this out into 

     A) pure renames uintXXX to blobXXX, can easily be verified (in reverse) with
        find -name \*.h -print0 | xargs -0 sed -i 's/blob256/uint256/g'
        find -name \*.cpp -print0 | xargs -0 sed -i 's/blob256/uint256/g'
        find -name \*.h -print0 | xargs -0 sed -i 's/blob160/uint160/g'
        find -name \*.cpp -print0 | xargs -0 sed -i 's/blob160/uint160/g'

     B) string conversions uint256("string") to blob256S("string")

     C) Added #includes and predeclared classes

     D) Necessary conversions between uint256 and blob256 **Focus reviewing here**

5. Remove now-unused methods from base_uint and blob160/blob256, eg GetHash, also remove unused uint160.


Would it break anything if we defined the ordering of blob256 as platform-dependent? That would allow using memcmp for this operator too.