stage2: implement structs in the frontend

New ZIR instructions:
 * struct_decl_packed
 * struct_decl_extern

New TZIR instruction: struct_field_ptr

Introduce `Module.Struct`. It uses `Value` to store default values and
abi alignments.

Implemented Sema.analyzeStructFieldPtr and zirStructDecl.

Some stuff I changed from `@panic("TODO")` to `log.warn("TODO")`.
It's becoming more clear that we need the lazy value mechanism soon;
Type is becoming unruly, and some of these functions have too much logic
given that they don't have any context for memory management or error
reporting.

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