Remove gas estimator from the compiler

Gas estimator built into the compiler (`solc --gas`) has not been actively maintained for some time now and has multiple bugs and missing features.

~We don't think it makes sense to put more effort into it so we'll be removing it in the next breaking version. There will be no deprecation warnings because as far as we can tell the estimator is rarely used in practice and there are no major tools that depend on it.~

This needs a deprecation warning.

The issue names the solc --gas compiler option but no files or tests. Start by locating the option's implementation and the project's existing deprecation-warning coverage; done means the requested warning is emitted for this option and the change is covered by appropriate tests.

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