Precompile for native token ERC-20 interface

[JamesHinshelwood]

The native token is special and has first-class support in our account model (as in all other EVM chains that I know of). Usually, developers need to handle this special case in their contracts. For example an exchange might need functions to swap tokens for other tokens, tokens for ZIL and ZIL for tokens.

To make life easier for developers, we should add a precompile which exposes an ERC-20-compatible interface to the chain's native token.

We'll have to think about how exactly to support allowances and events.

[JamesHinshelwood]

It probably makes sense to split this implementation into two:

• A stateless, but incomplete version which doesn't support allowances.
• A stateful and complete version later.

[JamesHinshelwood]

#804 has started doing this.