* anonymous decls have automatically generated names and symbols, and
participate in the same memory management as named decls.
* the Ref instruction is deleted
* the DeclRef instruction now takes a `[]const u8` and DeclRefStr takes
an arbitrary string instruction operand.
* introduce a `zir.Decl` type for ZIR Module decls which holds
content_hash and name - fields that are not needed for `zir.Inst`
which are created as part of semantic analysis. This improves the
function signatures of Module.zig and lowers memory usage.
* the Str instruction is now defined to create an anonymous Decl and
reference it.
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