add code coverage host functions

[praphael]

Change Description

Adds new host functions to support contract code coverage.

Change Type

Select ONE:

• [ ] Documentation

• [ ] Stability bug fix

• [ ] Other

• [x] Other - special case

Testing Changes

Select ANY that apply:

• [ ] New Tests

• [ ] Existing Tests

• [ ] Test Framework

• [ ] CI System

• [ ] Other

Consensus Changes

• [ ] Consensus Changes

API Changes

• [x] API Changes

Documentation Additions

• [ ] Documentation Additions

[praphael]

These new host functions should only be added to eosio-tester and rodeos not nodeos.

I'm not sure its possible with only an eosio-tester modification. Tester intrinsics only appear to work with test contracts. The test contract uses a special framework which in turn calls the normal contracts. It also has to define main(), which normal contracts don't. So I don't believe our normal contracts can be compiled as test contracts so they can use pure tester intrisics.

I have verified that the recent commit 7e6b809 works through the whole cycle,. and generates a reasonable code coverage report. We don't actually have to merge this in, we could keep it as a separate branch just for the purpose of using code coverage.