Breaking changes:
- The ChainConfig was exposed to tracers via VMContext passed in
`OnTxStart`. This is unnecessary specially looking through the lens of
live tracers as chain config remains the same throughout the lifetime of
the program. It was there so that native API-invoked tracers could
access it. So instead we moved it to the constructor of API tracers.
Non-breaking:
- Change the default config of the tracers to be `{}` instead of nil.
This way an extra nil check can be avoided.
Refactoring:
- Rename `supply` struct to `supplyTracer`.
- Un-export some hook definitions.
