Add a Field node for fields in a class. (#3332)

This replaces the use of `VarStorage` in this case.

Add an `UnboundFieldType` type as the type of a field, in cases where
it's referenced without an accompanying object.

Add a `BindName` node to describe the name binding performed for both
variables and fields so that we can handle them more uniformly.

toolchain/check/handle_function.cpp

@@ -178,7 +178,7 @@ auto HandleFunctionDefinitionStart(Context& context, Parse::Node parse_node)
   // Create the function scope and the entry block.
   context.return_scope_stack().push_back(decl_id);
   context.node_block_stack().Push();
-  context.PushScope();
+  context.PushScope(decl_id);
   context.AddCurrentCodeBlockToFunction();
 
   // Bring the parameters into scope.