Fixes several sign-comparison warnings.

- Easier for debugging (what opcode was 0x... again?)
- Clarifies that the opcodes are set in stone in the protocol, and signals that it is impossible to insert opcodes in between.

In ISO C++, the signedness of 'char' is undefined.  On some platforms (e.g.
ARM), 'char' is an unsigned type, but some of the code relies on 'char' being
signed (as it is on x86).  This is indicated by compiler warnings like this:

 bignum.h: In constructor 'CBigNum::CBigNum(char)':
 bignum.h:81:59: warning: comparison is always true due to limited range of data type [-Wtype-limits]

 util.cpp: In function 'bool IsHex(const string&)':
 util.cpp:427:28: warning: comparison is always false due to limited range of data type [-Wtype-limits]

In particular, IsHex erroneously returned true regardless of the input
characters, as long as the length of the string was a positive multiple of 2.

Note: For testing, it's possible using GCC to force char to be unsigned by
adding the -funsigned-char parameter to xCXXFLAGS.

This reverts commit 21d9f367.

so it takes a flag for how to interpret OP_EVAL.
Also increased IsStandard size of scriptSigs to 500 bytes, so
a 3-of-3 multisig transaction IsStandard.

OP_EVAL is a new opcode that evaluates an item on the stack as a script.
It enables a new type of bitcoin address that needs an arbitrarily
complex script to redeem.

Initial support for (a and b), (a or b), and 2-of-3 escrow
transactions (where a, b, and c are keys).

To a variation on:
// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2011 The Bitcoin developers

This is needed for the script unit tests.

Instead of conversion functions between pubkey/uint160/address in
base58.h, have a fully fledged class CBitcoinAddress (CAddress was
already taken) to represent addresses.

Make CKeyStore's interface work on uint160's instead of pubkeys, so
no separate global mapPubKeys is necessary anymore.

In the assert()s take advantage of the fact that string constants
("string") are effectively of type 'const char []', which when used in
an expression yield a non-NULL pointer.

An assertion that should always fail can thus be formulated as:
  assert(!"fail);

An assertion where a text message should be added to the expression can
be written as such:
  assert("message" && expression);
Signed-off-by: Giel van Schijndel <me@mortis.eu>

* A new class CKeyStore manages private keys, and script.cpp depends on access to CKeyStore.
* A new class CWallet extends CKeyStore, and contains all former wallet-specific globals; CWallet depends on script.cpp, not the other way around.
* Wallet-specific functions in CTransaction/CTxIn/CTxOut (GetDebit, GetCredit, GetChange, IsMine, IsFromMe), are moved to CWallet, taking their former 'this' argument as an explicit parameter
* CWalletTx objects know which CWallet they belong to, for convenience, so they have their own direct (and caching) GetDebit/... functions.
* Some code was moved from CWalletDB to CWallet, such as handling of reserve keys.
* Main.cpp keeps a set of all 'registered' wallets, which should be informed about updates to the block chain, and does not have any notion about any 'main' wallet. Function in main.cpp that require a wallet (such as GenerateCoins), take an explicit CWallet* argument.
* The actual CWallet instance used by the application is defined in init.cpp as "CWallet* pwalletMain". rpc.cpp and ui.cpp use this variable.
* Functions in main.cpp and db.cpp that are not used by other modules are marked static.
* The code for handling the 'submitorder' message is removed, as it not really compatible with the idea that a node is independent from the wallet(s) connected to it, and obsolete anyway.

This introduces two new source files, keystore.cpp and wallet.cpp with
corresponding headers. Code is moved from main and db, in a preparation
for a follow-up commit which introduces the classes CWallet and CKeyStore.