stage2: progress towards stage3

* The `@bitCast` workaround is removed in favor of `@ptrCast` properly
   doing element casting for slice element types. This required an
   enhancement both to stage1 and stage2.
 * stage1 incorrectly accepts `.{}` instead of `{}`. stage2 code that
   abused this is fixed.
 * Make some parameters comptime to support functions in switch
   expressions (as opposed to making them function pointers).
 * Avoid relying on local temporaries being mutable.
 * Workarounds for when stage1 and stage2 disagree on function pointer
   types.
 * Workaround recursive formatting bug with a `@panic("TODO")`.
 * Remove unreachable `else` prongs for some inferred error sets.

All in effort towards #89.

src/Module.zig

@@ -4593,7 +4593,7 @@ pub fn clearDecl(
                 .c => .{ .c = {} },
                 .wasm => .{ .wasm = link.File.Wasm.FnData.empty },
                 .spirv => .{ .spirv = .{} },
-                .nvptx => .{ .nvptx = .{} },
+                .nvptx => .{ .nvptx = {} },
             };
         }
         if (decl.getInnerNamespace()) |namespace| {
@@ -4975,7 +4975,7 @@ pub fn allocateNewDecl(
             .c => .{ .c = {} },
             .wasm => .{ .wasm = link.File.Wasm.FnData.empty },
             .spirv => .{ .spirv = .{} },
-            .nvptx => .{ .nvptx = .{} },
+            .nvptx => .{ .nvptx = {} },
         },
         .generation = 0,
         .is_pub = false,