self-hosted: implement Decl lookup

 * Take advantage of coercing anonymous struct literals to struct types.
 * Reworks Module to favor Zig source as the primary use case.
   Breaks ZIR compilation, which will have to be restored in a future commit.
 * Decl uses src_index rather then src, pointing to an AST Decl node
   index, or ZIR Module Decl index, rather than a byte offset.
 * ZIR instructions have an `analyzed_inst` field instead of Module
   having a hash table.
 * Module.Fn loses the `fn_type` field since it is redundant with
   its `owner_decl` `TypedValue` type.
 * Implement Type and Value copying. A ZIR Const instruction's TypedValue
   is copied to the Decl arena during analysis, which allows freeing the
   ZIR text instructions post-analysis.
 * Don't flush the ELF file if there are compilation errors.
 * Function return types allow arbitrarily complex expressions.
 * AST->ZIR for function calls and return statements.

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