Refactor semantic checking code into more files (#1097)

The semantic checking logic was all inside `slang-check.cpp` and as a result this was a monster file that was extremely hard to follow. This change splits `slang-check.cpp` into several smaller files, although some of the resulting files are still quite large.

This change attempts to be a copy-paste job as much as possible and does *not* perform any cleanup on naming, structure, duplication, etc. in the code it deal with. No function bodies or signatures have been touched.

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