compiler: change representation of closures

This changes the representation of closures in Zir and Sema. Rather than
a pair of instructions `closure_capture` and `closure_get`, the system
now works as follows:

* Each ZIR type declaration (`struct_decl` etc) contains a list of
  captures in the form of ZIR indices (or, for efficiency, direct
  references to parent captures). This is an ordered list; indexes into
  it are used to refer to captured values.
* The `extended(closure_get)` ZIR instruction refers to a value in this
  list via a 16-bit index (limiting this index to 16 bits allows us to
  store this in `extended`).
* `Module.Namespace` has a new field `captures` which contains the list
  of values captured in a given namespace. This is initialized based on
  the ZIR capture list whenever a type declaration is analyzed.

This change eliminates `CaptureScope` from semantic analysis, which is a
nice simplification; but the main motivation here is that this change is
a prerequisite for #18816.

src/InternPool.zig

@@ -1,7 +1,6 @@
 //! All interned objects have both a value and a type.
 //! This data structure is self-contained, with the following exceptions:
 //! * Module.Namespace has a pointer to Module.File
-//! * Module.Decl has a pointer to Module.CaptureScope
 
 /// Maps `Key` to `Index`. `Key` objects are not stored anywhere; they are
 /// constructed lazily.
@@ -6395,7 +6394,6 @@ fn finishFuncInstance(
         .@"addrspace" = fn_owner_decl.@"addrspace",
         .analysis = .complete,
         .zir_decl_index = fn_owner_decl.zir_decl_index,
-        .src_scope = fn_owner_decl.src_scope,
         .is_pub = fn_owner_decl.is_pub,
         .is_exported = fn_owner_decl.is_exported,
         .alive = true,
@@ -7891,6 +7889,7 @@ pub fn destroyNamespace(ip: *InternPool, gpa: Allocator, index: NamespaceIndex)
         .parent = undefined,
         .file_scope = undefined,
         .decl_index = undefined,
+        .captures = undefined,
     };
     ip.namespaces_free_list.append(gpa, index) catch {
         // In order to keep `destroyNamespace` a non-fallible function, we ignore memory