Expression-level coverage support

Describe the desired feature

This should be a fairly simple task because source maps already return offset and length of the expression in the source. We do line coverage with this function, which computes the line number https://github.com/ethereum/hevm/blob/6c876b24b61d2c8f8b9919cc668db9a03e6496c8/src/EVM/Dapp.hs#L179-L183. Instead, we can just use offset and length from SrcMap and color accordingly. For now, we should do it only for html and think how to do it with text and lcov formats (not sure if this is even supported).