Make source maps byte-offset based instead of opcode-offset

[chriseth]

The source maps currently contain one item per opcode, instead of one item per byte in the assembly. This kind of makes sense because it allows the source maps to be flexible about which push opcode is used for a constant, but it makes it also more difficult because the "pseudo-opcode" AssignImmutable results in multiple opcodes in the end. To make things worse: As soon as one uses verbatim, it is taken as a single opcode by the compiler, but when just seeing the bytecode, it is impossible to tell whether a sequence of opcodes came from a single verbatim or multiple opcodes.

Because of that I would propose to make the source maps byte-based starting from Solidity 0.9.0.