This allows to reuse it across different packages.
testchain can't be used because of circular dependencies.
Init() is not changed except for filepath.Join() use instead of direct string
appends which is a better approach anyway. rootpath is required because
current directory will change from package to package.
Compiler forces notification parameters to match the one specified in
manifest, and manifest of `test_contract.go` tells that `to` parameter
type is hash160:
```
event 'Transfer' should have 'Hash160' as type of 1 parameter, got: ByteArray
```
We need to store NEO balance's LastUpdateHeight before GAS mint,
because mint can call onNEP17Payment and onNEP17Payment can call NEO
transfer which also calls GAS mint. Storing balance height allows to
avoid recursion.
Allow to invoke methods by offset:
1. Every invoked contract must have manifest.
2. Check arguments count on invocation.
3. Change AppCall to a regular syscall.
4. Add test suite for `System.Contract.Call`.
