This commit contains everything from the copy-elision-2
branch that does not have to do with copy elision directly,
but is generally useful for master branch.
* All const values know their parents, when applicable, not
just structs and unions.
* Null pointers in const values are represented explicitly,
rather than as a HardCodedAddr value of 0.
* Rename "maybe" to "optional" in various code locations.
* Separate DeclVarSrc and DeclVarGen
* Separate PtrCastSrc and PtrCastGen
* Separate CmpxchgSrc and CmpxchgGen
* Represent optional error set as an integer, using the 0 value.
In a const value, it uses nullptr.
* Introduce type_has_one_possible_value and use it where applicable.
* Fix debug builds not setting memory to 0xaa when storing
undefined.
* Separate the type of a variable from the const value of a variable.
* Use copy_const_val where appropriate.
* Rearrange structs to pack data more efficiently.
* Move test/cases/* to test/behavior/*
* Use `std.debug.assertOrPanic` in behavior tests instead of
`std.debug.assert`.
* Fix outdated slice syntax in docs.
