RequirementMachine: The great renaming

[slavapestov]

• "Atoms" should be called "symbols" because that's a much more common term for this, and there's little opportunity for confusion with linker symbols here.
• The "equivalence class map" should be called the "property map" since it collects information about property-like rules, which involve property symbols, and there aren't really any equivalence classes with a formal equivalence relation here.
• RequirementMachine.h can live in lib/AST/RequirementMachine/ since it's only used by GenericSignature.cpp and ASTContext.cpp, and I don't want other usages to creep in; the rewrite system should be entirely abstracted away by "nice" APIs that traffic in AST objects.
• Due to the above, we don't have to use the PIMPL pattern, it just adds unnecessary indirection.
• Finally, it's called the shortlex order, not lexshort. Fix that.

[slavapestov]

@swift-ci Please smoke test