stage2: introduce new ZIR instruction: arg

 * AstGen: LocalVal and LocalPtr use string table indexes for their
   names. This is more efficient because local variable declarations do
   need to include the variable names so that semantic analysis can emit
   a compile error if a declaration is shadowed. So we take advantage of
   this fact by comparing string table indexes when resolving names.

 * The arg ZIR instructions are needed for the above reasoning, as well
   as to emit equivalent AIR instructions for debug info.
   Now that we have these arg instructions, get rid of the special
   `Zir.Inst.Ref` range for parameters. ZIR instructions now refer
   to the arg instructions for parameters.

 * Move identAsString and strLitAsString from Module.GenZir to AstGen
   where they belong.

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