* modify stage2 tests so that only 1 uses _start and the rest use
   pub fn main
 * modify stage2 CBE tests so that only 1 uses pub export main and the
   rest use pub fn main

 * get the test runner and `zig test` working
 * get behavior tests passing for stage2

 * use a hash map for instructions because the array is too big
   - no, actually modify the Zir.Inst.Ref strategy so that each decl gets
     their indexes starting at 0 so that we can use an array to store Sema
     results rather than a map.

 * implement the new AstGen compile errors

 * get rid of failed_root_src_file
 * get rid of Scope.DeclRef
 * get rid of NameHash
 * handle decl collision with usingnamespace
 * the decl doing the looking up needs to create a decl dependency
   on each usingnamespace decl
 * handle usingnamespace cycles

 * compile error for return inside defer expression

 * when block has noreturn statement
   - avoid emitting defers
   - compile error for unreachable code
 
 * detect `return error.Foo` and emit ZIR that unconditionally generates errdefers
 * `return`: check return operand and generate errdefers if necessary

 * have failed_trees and just put the file in there
   - this way we can emit all the parse errors not just the first one
   - but maybe we want just the first one?

 * need a var decl zir instruction which includes the name because we need to do the
   compile error for a local shadowing a decl with Sema looking up the decl name.
   - this means LocalVal and LocalPtr should use the string table

 * sort compile errors before presenting them to eliminate nondeterministic error reporting

 * AstGen: add result location pointers to function calls
 * nested function decl: how to refer to params?

 * fix the commented out behavior test regarding function alignment
   - not sure why this happened, it's stage1 code??
   - search the behavior test diff for "TODO"

 * memory efficiency: add another representation for structs which use
   natural alignment for fields and do not have any comptime fields. this
   will save 16 bytes per struct field in the compilation.

 * extern "foo" for vars

 * use ZIR memory for decl names where possible and also for keys
   - this will require more sophisticated changelist detection which does some
     pre-emptive deletion of decls from the parent namespace

 * better anonymous Decl naming convention
   - avoid the global atomic integer for the number because of contention

 * AstGen memory leak with `block_gz.labeled_store_to_block_ptr_list.append`

 * in SwitchProng resolve, make sure AST tree gets loaded.
   It will be unloaded if using cached ZIR.
 
 * make AstGen smart enough to omit elided store_to_block_ptr instructions

 * repl: if you try `run` with -ofmt=c you get an access denied error because it
   tries to execute the .c file as a child process instead of executing `zig run`
   on it.