compiler: split Decl into Nav and Cau

The type `Zcu.Decl` in the compiler is problematic: over time it has
gained many responsibilities. Every source declaration, container type,
generic instantiation, and `@extern` has a `Decl`. The functions of
these `Decl`s are in some cases entirely disjoint.

After careful analysis, I determined that the two main responsibilities
of `Decl` are as follows:
* A `Decl` acts as the "subject" of semantic analysis at comptime. A
  single unit of analysis is either a runtime function body, or a
  `Decl`. It registers incremental dependencies, tracks analysis errors,
  etc.
* A `Decl` acts as a "global variable": a pointer to it is consistent,
  and it may be lowered to a specific symbol by the codegen backend.

This commit eliminates `Decl` and introduces new types to model these
responsibilities: `Cau` (Comptime Analysis Unit) and `Nav` (Named
Addressable Value).

Every source declaration, and every container type requiring resolution
(so *not* including `opaque`), has a `Cau`. For a source declaration,
this `Cau` performs the resolution of its value. (When ziglang#131 is
implemented, it is unsolved whether type and value resolution will share
a `Cau` or have two distinct `Cau`s.) For a type, this `Cau` is the
context in which type resolution occurs.

Every non-`comptime` source declaration, every generic instantiation,
and every distinct `extern` has a `Nav`. These are sent to codegen/link:
the backends by definition do not care about `Cau`s.

This commit has some minor technically-breaking changes surrounding
`usingnamespace`. I don't think they'll impact anyone, since the changes
are fixes around semantics which were previously inconsistent (the
behavior changed depending on hashmap iteration order!).

Aside from that, this changeset has no user-facing changes. Instead, it
is an internal refactor which makes it easier to correctly model the
responsibilities of different objects, particularly regarding
incremental compilation. The performance impact should be negligible,
but I will take measurements before merging this work into `master`.

src/Sema/bitcast.zig

@@ -254,7 +254,7 @@ const UnpackValueBits = struct {
             .error_set_type,
             .inferred_error_set_type,
             .variable,
-            .extern_func,
+            .@"extern",
             .func,
             .err,
             .error_union,

src/Sema/comptime_ptr_access.zig

@@ -217,15 +217,23 @@ fn loadComptimePtrInner(
     };
 
     const base_val: MutableValue = switch (ptr.base_addr) {
-        .decl => |decl_index| val: {
-            try sema.declareDependency(.{ .decl_val = decl_index });
-            try sema.ensureDeclAnalyzed(decl_index);
-            const decl = zcu.declPtr(decl_index);
-            if (decl.val.getVariable(zcu) != null) return .runtime_load;
-            break :val .{ .interned = decl.val.toIntern() };
+        .nav => |nav| val: {
+            try sema.declareDependency(.{ .nav_val = nav });
+            try sema.ensureNavResolved(src, nav);
+            const val = ip.getNav(nav).status.resolved.val;
+            switch (ip.indexToKey(val)) {
+                .variable => return .runtime_load,
+                // We let `.@"extern"` through here if it's a function.
+                // This allows you to alias `extern fn`s.
+                .@"extern" => |e| if (Type.fromInterned(e.ty).zigTypeTag(zcu) == .Fn)
+                    break :val .{ .interned = val }
+                else
+                    return .runtime_load,
+                else => break :val .{ .interned = val },
+            }
         },
         .comptime_alloc => |alloc_index| sema.getComptimeAlloc(alloc_index).val,
-        .anon_decl => |anon_decl| .{ .interned = anon_decl.val },
+        .uav => |uav| .{ .interned = uav.val },
         .comptime_field => |val| .{ .interned = val },
         .int => return .runtime_load,
         .eu_payload => |base_ptr_ip| val: {
@@ -580,7 +588,7 @@ fn prepareComptimePtrStore(
 
     // `base_strat` will not be an error case.
     const base_strat: ComptimeStoreStrategy = switch (ptr.base_addr) {
-        .decl, .anon_decl, .int => return .runtime_store,
+        .nav, .uav, .int => return .runtime_store,
         .comptime_field => return .comptime_field,
         .comptime_alloc => |alloc_index| .{ .direct = .{
             .alloc = alloc_index,