All transactions must declare a list of permissions they utilize. The
chain now checks that the signatures are present to satisfy these
permissions, at least theoretically (only partially tested). As the
transaction is evaluated and applied, the message handlers will check
that the required permissions were declared on the transaction.

Also, define the logic to check that an authority is satisfied (only this
part is tested so far)

TODO: Test that transactions are rejected if they do not bear sufficient
signatures
TODO: Make message handlers check the declared permissions are sufficient,
and reject the transaction if they are not.

In original design all messages passed between contracts would consist
of well defined types, these types would be used with code generation to
parse message data and then pass rich Wren objects to user defined
message handlers.  The switch to Web Assembly made this irrelevent and
counter productive.

Going forward applications will be able to define and publish their own
ABI and serialization techniques. This will make the core platform simpler.

Fixed code caching

OK, so this turned out to be a **massive** refactor that I thought I'd
never finish, and no, I don't remember everything that changed here, but
I think it all worked out to be better than before, and almost all of it
was fairly necessary/the best of available options. Whew. I am SO glad
this is finally done.

High level goal: Continue refining the native contracts (ref #15),
specifically
walling off their state into their own contract DBs rather than globally
shared (read: stuff that race conditions are made of) general blockchain
state.

The first real change I made in this commit, which precipitated the
avalanche of changes that followed before the project became consistent
again, was to move the account balance out of account_object (globally
shared) to an object known only to the Eos Contract, thus eliminating
the possibility of race conditions/nondeterministic behavior.

The trouble is, balances are set at genesis. Genesis is processed by
chain_controller. chain_controller cannot know about balances, since
those are an abstraction defined on top of the Eos contract, which is an
abstraction defined on top of chain_controller. So all of genesis had to
be reimagined, and quite a lot of architectural changes had to be made
in order to create a logically consistent solution.

Changes (probably incomplete):
 - Fix up notifications within the native contract to support
precondition validation as well as application

 - Add notify handlers for CreateAccount to Eos and Staked Balance
contracts

 - Move account's liquid balance of EOS from account_object to
BalanceObject

 - Replace {producer,account}_object::id_type with AccountName most
everywhere except block_header, which still contains a
producer_object::id_type (potentially in violation of protocol
standards, but I want to confirm that before fixing it)
   - Reason: The name is not significantly slower, as it's fixed length so
no heap allocs, and it simplifies the code in quite a few places by
allowing us to look up objects directly rather than indirectly by
looking up an ID to get an intermediate object to get a handle for the
object we really wanted

 - Replace native_system_contract_plugin with native_contract library
   - Reason: The plugin was getting in the way. The native system
contract C++ implementation is simply too fundamental unless/until we
have a scripted implementation that works until the native
implementation gets installed

 - Completely reimagine genesis initialization, taking it largely out of
the hands of chain_controller and putting it in the hands of
native_contract
   - Reason: chain_controller understands relatively little about
genesis. It understands global_property_object and producer_object, but
not BalanceObject or StakedBalanceObject, etc... It also doesn't
understand the native_contract, and things like installing the native
contract, setting up accounts/balances, etc. all need to be handled by
something... native_contract is the most logical place to put it.

Sorry for the enormous commit... alas, this was the first time I got it
all building again and passing tests in days.

Replace monolithic implementation file system_contract_impl.hpp with
topic-specific implementations in separate files with clean header/
implementation separation. Use structs to namespace the various message
handlers rather than Message_ prefixes.